py3: suppress the output of open() using `and None`

This patch suppresses the output of open() on Python 3 as it does not return any
output on Python 2.

This makes test-diffstat.t pass on Python 3.5

Differential Revision: https://phab.mercurial-scm.org/D3128

tests/test-diffstat.t

@@ -35,7 +35,7 b' Narrow diffstat:'
 
   $ hg ci -m appenda
 
-  >>> open("c", "wb").write(b"\0")
+  >>> open("c", "wb").write(b"\0") and None
   $ touch d
   $ hg add c d
 
@@ -54,7 +54,7 b' Binary git diffstat:'
 
   $ hg ci -m createb
 
-  >>> open("file with spaces", "wb").write(b"\0")
+  >>> open("file with spaces", "wb").write(b"\0") and None
   $ hg add "file with spaces"
 
 Filename with spaces diffstat: