I'm getting a similar segfault during the test_inplace rule, on lxml 2.4.3, python 3.6.6, on aarch64, compiled with gcc-8.2.0 on glibc-2.27.

   ... <snip>
   1584/1822o(j86.9%):ttxtt(validation)ingios(...est_relaxng.ETreeRelaxNGTestCase)
   1605/1822v(l88.1%):.txtt(sax)to_pulldoml(lxml.tests.test_sax.ETreeSaxTestCase))
   1611/1822s(x88.4%): txtt(validation)invalid_schema_namespace.(...atronTestCase)
   1681/1822v(l92.3%):.txtt(validation)esolvers_rooto(...LSchemaResolversTestCase)
   1734/1822v(l95.2%):.xpathx(...or)mpile_nst(...luator.ETreeETXPathClassTestCase)
   1735/1822l(m95.2%):.txtt(xpathxslt)ator.xpath
   1821/1822x(a99.9%):ttxtt(extensions)lement_raises(...reeXSLTExtElementTestCase)
   1822/1822e(100.0%):.txt (xpathxslt)
   Doctest: xpathxslt.txt
   ----------------------------------------------------------------------
   Ran 1822 tests in 95.962s
   
   OK
   Comparing with ElementTree 1.3.0
   
   TESTED VERSION: 4.2.3
       Python:           sys.version_info(major=2, minor=7, micro=15, releaselevel='final', serial=0)
       lxml.etree:       (4, 2, 3, 0)
       libxml used:      (2, 9, 8)
       libxml compiled:  (2, 9, 8)
       libxslt used:     (1, 1, 32)
       libxslt compiled: (1, 1, 32)
   
   make[2]: *** [Makefile:71: test_inplace] Segmentation fault (core dumped)

Note that I pulled in the change from https://mailman-mail5.webfaction.com/pipermail/lxml/20170113/015734.html, because the coverage report code took about an hour and sucked up ~2GB of memory, only to spit out a spurious error about the first "cimport" statement it found.  The same segfault happens with or without that change though.

My backtrace looks like this:

#0  0x0000007f8ac12350 in PyDict_GetItem () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#1  0x0000007f8abfc24c in PyFrame_New () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#2  0x0000007f894975d8 in __Pyx_TraceSetupAndCall (code=code@entry=0x7f897cf518 <__pyx_frame_code.70765>, frame=frame@entry=0x7ff0a77c28, tstate=0x1d7032f0, funcname=funcname@entry=0x7f89745f48 "__dealloc__", 
    srcfile=srcfile@entry=0x7f8973fbd8 "src/lxml/parser.pxi", firstlineno=firstlineno@entry=530) at src/lxml/etree.c:257932
        type = <optimized out>
        value = <optimized out>
        traceback = <optimized out>
        retval = <optimized out>
#3  0x0000007f894ef5e8 in __pyx_pf_4lxml_5etree_14_ParserContext_2__dealloc__ (__pyx_v_self=0x7f848b1750) at src/lxml/etree.c:114106
        tstate = <optimized out>
        __pyx_frame = 0x0
        __pyx_t_2 = <optimized out>
        __Pyx_use_tracing = 0
        __pyx_t_1 = <optimized out>
        __pyx_t_3 = <optimized out>
        __pyx_frame_code = 0x7f84b3c4b0
        __pyx_frame = <optimized out>
        __Pyx_use_tracing = <optimized out>
        __pyx_t_1 = <optimized out>
        __pyx_t_2 = <optimized out>
        __pyx_t_3 = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
---Type <return> to continue, or q <return> to quit--- 
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
        ret = <optimized out>
        tstate = <optimized out>
        state = <optimized out>
        tstate = <optimized out>
#4  __pyx_pw_4lxml_5etree_14_ParserContext_3__dealloc__ (__pyx_v_self=0x7f848b1750) at src/lxml/etree.c:48557
No locals.
#5  __pyx_tp_dealloc_4lxml_5etree__ParserContext (o=0x7f848b1750) at src/lxml/etree.c:35990
        etype = 0x0
        eval = 0x0
        etb = 0x0
        p = 0x7f848b1750
#6  0x0000007f89468580 in __pyx_tp_dealloc_4lxml_5etree__BaseParser (o=0x7f852ee190) at src/lxml/etree.c:232721
        _py_tmp = <optimized out>
        p = 0x7f852ee190
#7  0x0000007f85189edc in __pyx_module_cleanup (self=<optimized out>) at src/lxml/objectify.c:39696
        _py_tmp = <optimized out>
#8  __pyx_module_cleanup_atexit (module=<optimized out>, unused=<optimized out>) at src/lxml/objectify.c:43062
No locals.
#9  0x0000007f8ac81c50 in PyEval_EvalFrameEx () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#10 0x0000007f8ac851d0 in PyEval_EvalCodeEx () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#11 0x0000007f8abfcc5c in ?? () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#12 0x0000007f8abd131c in PyObject_Call () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#13 0x0000007f8ac7b664 in PyEval_CallObjectWithKeywords () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#14 0x0000007f8acaac70 in Py_Finalize () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#15 0x0000007f8acaa658 in Py_Exit () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#16 0x0000007f8acaa740 in ?? () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#17 0x0000007f8acaa958 in PyErr_PrintEx () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#18 0x0000007f8acab4bc in PyRun_SimpleFileExFlags () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#19 0x0000007f8acbdf98 in Py_Main () from /usr/lib/aarch64-linux-gnu/libpython2.7.so.1.0
No symbol table info available.
#20 0x0000007f8a91b8e4 in __libc_start_main () from /lib/aarch64-linux-gnu/libc.so.6
No symbol table info available.
#21 0x0000000000400880 in _start ()
No symbol table info available.
Backtrace stopped: previous frame identical to this frame (corrupt stack?)