D2158: py3: use raw string for open() mode

[yuja (Yuya Nishihara)]

yuja added a comment.


  I wanna leave it with no prefix as possible. I think we'll have to bulk-replace
  `''`s to `b''`s at some point so we can leverage static analysis tools, where
  `r''` will be unneeded.

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2158

To: indygreg, #hg-reviewers
Cc: yuja, pulkit, mercurial-devel