Absolute path allowed from ElementTree but fails to find root element

[masklinn]

While absolute ElementPaths are forbidden on Element, lxml does allow them on ElementTree. However they are incorrect, as they fail to find the root element itself (unlike xpath).

CPython's elementtree has a similar issue however it triggers a FutureWarning

Test script:
```
import contextlib
from xml.etree import ElementTree as ET
from lxml import etree

doc = ET.fromstring("<a><b/></a>")
# raises SyntaxError
with contextlib.suppress(SyntaxError):
    print(doc.find("/a"))
    raise Exception()
# triggers FutureWarning: This search is broken in 1.3 and earlier, and will be fixed in a future version.
# does not find anything
print(ET.ElementTree(doc).find("/a"))

doc = etree.fromstring("<a><b/></a>")

# raises SyntaxError
with contextlib.suppress(SyntaxError):
    print(doc.find("/a"))
    raise Exception()

# doesn't find anything, no warning
print(doc.getroottree().find("/a"))
# by comparison xpath finds the root, whether from an Element or an
# ElementTree
print(doc.xpath("/a"))
print(doc.getroottree().xpath("/a"))
```

system info:
Python : sys.version_info(major=3, minor=12, micro=0, releaselevel='final', serial=0)
lxml.etree : (4, 9, 3, 0)
libxml used : (2, 10, 3)
libxml compiled : (2, 10, 3)
libxslt used : (1, 1, 38)
libxslt compiled : (1, 1, 38)

[scoder (scoder)]

I agree that this is incorrect. I added the same FutureWarning as in Python's xml.etree. Changing this requires a deprecation period, and preferably CPython going there first.

https://github.com/lxml/lxml/commit/3a8caecc0f94d8e00ab116a4b32e57675140eb4d