gh-156495: Compare normalized prefixes in site._venv()

[Gronoxx]

Fixes #156495.

site._venv() compares sys.prefix against a site_prefix built with os.path.abspath(), using string equality. abspath() normalizes and getpath does not, so a venv interpreter started through a path such as ../venv/bin/python reports two spurious RuntimeWarnings about prefixes that name the same directory.

This compares normalized forms instead. No observable value changes: sys.prefix, sys.exec_prefix and sys.executable are untouched, and the message still reports the original value. Only the condition for warning changes, and genuine mismatches still warn, including the one from #154160.

The check itself came from GH-126987 (gh-126985), by Filipe Laíns, which replaced the earlier sys.prefix = sys.exec_prefix = site_prefix assignment. The abspath() on the other side has been there since #60723 (bpo-16519), reported by Christian Heimes and fixed by Vinay Sajip, added so that invoking the interpreter through a relative path would work. Both do what they were meant to do. Only the string comparison between them is too strict.

Two tests are added, since there was no coverage of this warning in test_site.py or test_venv.py. The first fails without the change. The second pins down a genuine mismatch, so that a future change cannot silence the check altogether.

Verification

Bisected on clean venvs, invoking ../v/bin/python from a sibling directory:

version	warnings	sys.prefix	sys.executable
3.11.15	0	/tmp/v	/tmp/sub/../v/bin/python
3.12.13	0	/tmp/v	/tmp/sub/../v/bin/python
3.13.15	0	/tmp/v	/tmp/sub/../v/bin/python
3.14.7	2	/tmp/sub/../v	/tmp/sub/../v/bin/python
3.15.0rc1	2	/tmp/sub/../v	/tmp/sub/../v/bin/python

sys.executable is identical throughout. Only sys.prefix and the check are new.

Every case, measured on this branch with and without the change:

case	before	after
../v/bin/python from a sibling directory	2	0
the same with env -i (no PATH in the environment)	2	0
relative PATH entry (the #154160 symptom)	2	2
absolute invocation	0	0
//tmp/v/bin/python	0	0
.. in the middle of the path	0	0
pyvenv.cfg next to the interpreter	2	2

Test suite, before and after, on test_venv test_site test_sys test_getpath test_embed test_sysconfig test_cmd_line test_cmd_line_script test_import test_frozen test_posixpath test_ntpath: 12 of 12 modules pass either way, 874 tests before and 876 after, the difference being the two added here.

os.path.normpath is _path_normpath, a C builtin, at 549 ns per call. That is roughly 1 µs per interpreter start, and only inside a venv. I could not measure it above noise end to end.

• Issue: site emits two spurious RuntimeWarnings when a venv interpreter is started via a non-normalized path (3.14 regression) #156495

[python-cla-bot]

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