tests: fix run-tests environment cleanup on Python 3

Differential Revision: https://phab.mercurial-scm.org/D2521

tests/run-tests.py

@@ -1087,7 +1087,7 b' class Test(unittest.TestCase):'
                 del env[k]
 
         # unset env related to hooks
-        for k in env.keys():
+        for k in list(env):
             if k.startswith('HG_'):
                 del env[k]