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

contrib/check-py3-compat.py

@@ -97,6 +97,15 b" if __name__ == '__main__':"
     if sys.version_info[0] == 2:
         fn = check_compat_py2
     else:
+        # check_compat_py3 will import every filename we specify as long as it
+        # starts with one of a few prefixes. It does this by converting
+        # specified filenames like 'mercurial/foo.py' to 'mercurial.foo' and
+        # importing that. When running standalone (not as part of a test), this
+        # means we actually import the installed versions, not the files we just
+        # specified. When running as test-check-py3-compat.t, we technically
+        # would import the correct paths, but it's cleaner to have both cases
+        # use the same import logic.
+        sys.path.insert(0, '.')
         fn = check_compat_py3
 
     for f in sys.argv[1:]: