py3: make sure we write bytes to file

# skip-blame because just b'' prefix

Differential Revision: https://phab.mercurial-scm.org/D3220

tests/test-mq.t

@@ -1126,7 +1126,7 create a git binary patch
   $ cat > writebin.py <<EOF
   > import sys
   > path = sys.argv[1]
-  > open(path, 'wb').write('BIN\x00ARY')
+  > open(path, 'wb').write(b'BIN\x00ARY')
   > EOF
   $ $PYTHON writebin.py bucephalus