Our supplied PYTHONPATH must stay first in the list so the installed mercurial
does not interfere with us.

--- mercurial-6.9.1/tests/run-tests.py.orig
+++ mercurial-6.9.1/tests/run-tests.py
@@ -3462,7 +3462,7 @@
         # are in /opt/subversion.)
         oldpypath = osenvironb.get(IMPL_PATH)
         if oldpypath:
-            pypath.append(oldpypath)
+            pypath.insert(0, oldpypath)
         osenvironb[IMPL_PATH] = sepb.join(pypath)
 
         os.environ["HGTEST_BASE_HGMODULEPOLICY"] = os.environ.get(