check-py3-compat: use an absolute path in sys.path

The idea and rationale is similar to https://phab.mercurial-scm.org/D12599
(landed as 1b6e381521c5).

Differential Revision: https://phab.mercurial-scm.org/D12621

contrib/check-py3-compat.py

@@ -105,7 +105,7 b" if __name__ == '__main__':"
         # 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, '.')
+        sys.path.insert(0, os.getcwd())
         fn = check_compat_py3
 
     for f in sys.argv[1:]: