lfs: use str for the open() mode when opening a blob for py3 The other fix for this was to leave the mode as bytes, and import `pycompat.open()` like a bunch of other modules do. But I think it's confusing to still use bytes at the python boundary, and obviously error prone. Grepping for ` open\(.+, ['"][a-z]+['"]\)` and ` open\(.+, b['"][a-z]+['"]\)` outside of `tests`, there are 51 and 87 uses respectively, so it's not like this is a rare direct usage. Differential Revision: https://phab.mercurial-scm.org/D8099