tests: fix printenv script on Python 3

Differential Revision: https://phab.mercurial-scm.org/D3725

tests/printenv.py

@@ -26,6 +26,7 b' except ImportError:'
 
 exitcode = 0
 out = sys.stdout
+out = getattr(out, 'buffer', out)
 
 name = sys.argv[1]
 if len(sys.argv) > 2: