py3: add `and None` to suppress return values of .write() calls
.write() calls don't return anything on Python2, so we need to make sure we
suppress that on py3 too.
This makes the test pass on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D3793
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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