tests: make check-py3-compat.py actually load the specified files correctly For most uses, this change is essentially a no-op, as this script is generally only run by test-check-py3-compat.t, which will already put `$TESTDIR/..` in `$PYTHONPATH`. When running outside of tests, however, `$PYTHONPATH` is likely not set, causing check-py3-compat.py to parse the file from the repo, but then import the installed version, and raise any errors about the installed version, not the one currently in the repo. Additionally, this helps users (like me) who have a strange set up where their home directory (and thus their hg repos) happen to be in a subdirectory of sys.prefix (which is /usr on my system). Since the '.' entry added to sys.path takes precedence over the absolute path of `$TESTDIR/..` in `$PYTHONPATH`, the path to the modules that it imports (and that show up in any stack trace) are *relative*, meaning that we don't detect them as starting with `sys.prefix`. Sample non-test invocation, and the difference this change makes (the path for 'error at <path>:<line>' is correct now):: Before: ``` $ python3 contrib/check-py3-compat.py mercurial/win*.py mercurial/win32.py: error importing: <ValueError> _type_ 'v' not supported (error at check-py3-compat.py:65) mercurial/windows.py: error importing: <ModuleNotFoundError> No module named 'msvcrt' (error at check-py3-compat.py:65) ``` After: ``` $ python3 contrib/check-py3-compat.py mercurial/win*.py mercurial/win32.py: error importing: <ValueError> _type_ 'v' not supported (error at win32.py:11) mercurial/windows.py: error importing: <ModuleNotFoundError> No module named 'msvcrt' (error at windows.py:12) ``` Differential Revision: https://phab.mercurial-scm.org/D8814