py3: make sure we write bytes in a file open in bytes mode

# skip-blame just b'' prefix

Differential Revision: https://phab.mercurial-scm.org/D2519

tests/test-grep.t

@@ -271,7 +271,7 b" test substring match: '^' should only ma"
 
 match in last "line" without newline
 
-  $ $PYTHON -c 'fp = open("noeol", "wb"); fp.write("no infinite loop"); fp.close();'
+  $ $PYTHON -c 'fp = open("noeol", "wb"); fp.write(b"no infinite loop"); fp.close();'
   $ hg ci -Amnoeol
   adding noeol
   $ hg grep loop