pycompat: fix a bytes vs str issue in `unnamedtempfile()`

This seems trivially correct, though the only two uses I found both took this
path.  So I'm guessing we're missing test coverage.

Differential Revision: https://phab.mercurial-scm.org/D10129

mercurial/pycompat.py

@@ -510,7 +510,7 b" def mkstemp(suffix=b'', prefix=b'tmp', d"
 # This wrapper file are always open in byte mode.
 def unnamedtempfile(mode=None, *args, **kwargs):
     if mode is None:
-        mode = b'w+b'
+        mode = 'w+b'
     else:
         mode = sysstr(mode)
     assert 'b' in mode