py3: suppress the value returned by .write() calls

These return values are not present on Python 2.

Differential Revision: https://phab.mercurial-scm.org/D3563

tests/test-status.t

@@ -465,12 +465,12 b" hg status of binary file starting with '"
 
   $ hg init repo5
   $ cd repo5
-  >>> open("010a", r"wb").write(b"\1\nfoo")
+  >>> open("010a", r"wb").write(b"\1\nfoo") and None
   $ hg ci -q -A -m 'initial checkin'
   $ hg status -A
   C 010a
 
-  >>> open("010a", r"wb").write(b"\1\nbar")
+  >>> open("010a", r"wb").write(b"\1\nbar") and None
   $ hg status -A
   M 010a
   $ hg ci -q -m 'modify 010a'