printenv: port to python3

printenv: port to python3

Differential Revision: https://phab.mercurial-scm.org/D2290

--- a/tests/printenv.py	Thu Feb 15 09:18:20 2018 -0500
+++ b/tests/printenv.py	Sat Feb 17 00:28:24 2018 -0500
@@ -35,7 +35,7 @@
 
 # variables with empty values may not exist on all platforms, filter
 # them now for portability sake.
-env = [(k, v) for k, v in os.environ.iteritems()
+env = [(k, v) for k, v in os.environ.items()
        if k.startswith("HG_") and v]
 env.sort()