tests: fix test-check-module-imports.t broken by D9150 stable

tests: fix test-check-module-imports.t broken by D9150

Differential Revision: https://phab.mercurial-scm.org/D9177

--- a/tests/test-doctest.py	Thu Oct 08 13:16:14 2020 -0700
+++ b/tests/test-doctest.py	Thu Oct 08 13:45:56 2020 -0700
@@ -4,7 +4,7 @@
 from __future__ import print_function
 
 import doctest
-import os.path
+import os
 import re
 import subprocess
 import sys