pypy ctypes.pythonapi has no symbol Py_FileSystemDefaultEncoding

[Wouter van Heyst]

The check in bzrlib._patch_filesystem_default_encoding does:

    pythonapi = getattr(ctypes, "pythonapi", None)
    if pythonapi is None:
        # Not CPython ctypes implementation
        return

and if that succeeds it tries:

   old_ptr = ctypes.c_void_p.in_dll(pythonapi, "Py_FileSystemDefaultEncoding")

Unfortunately pypy (since 1.6 at least) passes the check but fails the symbol lookup.

This can be triggered for example in bzrlib.tests.blackbox.test_locale.TestLocale.test_log_C

    Traceback (most recent call last):
      File "app_main.py", line 51, in run_toplevel
      File "/home/larstiq/src/bzr/bzr-pypy/bzr", line 74, in <module>
        import bzrlib
      File "/home/larstiq/src/bzr/bzr-pypy/bzrlib/__init__.py", line 171, in <module>
        _fs_enc = _patch_filesystem_default_encoding(sys._bzr_default_fs_enc)
      File "/home/larstiq/src/bzr/bzr-pypy/bzrlib/__init__.py", line 157, in _patch_filesystem_default_encoding
        old_ptr = ctypes.c_void_p.in_dll(pythonapi, "Py_FileSystemDefaultEncoding")
      File "/home/larstiq/src/pypy-1.7/lib_pypy/_ctypes/basics.py", line 85, in in_dll
        return self.from_address(dll._handle.getaddressindll(name))
    ValueError: No symbol Py_FileSystemDefaultEncoding found in library <None>

I'm not sure what the right approach is here. Can we depend on all CPython instances not raising the ValueError?
Check for known non-CPythons?