python3: use format-source to run byteify-strings in .py files
Using the format-source extension smooth out the pain of merging after
auto-formatting.
This change makes all of the Evolve test suite pass under python3 and has
added benefit of being 100% automated using mercurial's `byteify-strings`
script version 1.0 (revision 11498aa91c036c6d70f7ac5ee5af2664a84a1130).
How to benefit from the help of format-source is explained in the README.
python3: enforce byte prefix for vfs.open()
Changeset ea8da5aa23c6e723d7f0006387a1a20eef58fc9a added raw prefix to open(),
but also to vfs.open(). The latter only handles bytestring arguments.