.arcconfig

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

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