The problem with leaving pytype to its own devices here was that for functions
that returned a bytestr, pytype inferred `Union[bytes, int]`.  It now accepts
that it can be treated as plain bytes.

I wasn't able to figure out the arg type for `__getitem__`- `SupportsIndex`
(which PyCharm indicated is how the superclass function is typed) got flagged:

    File "/mnt/c/Users/Matt/hg/mercurial/pycompat.py", line 236, in __getitem__:
        unsupported operand type(s) for item retrieval: bytestr and SupportsIndex [unsupported-operands]
      Function __getitem__ on bytestr expects int

But some caller got flagged when I marked it as `int`.

There's some minor spillover problems elsewhere- pytype doesn't seem to
recognize that `bytes.startswith()` can optionally take a 3rd and 4th arg, so
those few places have the warning disabled.  It also flags where the tar API is
being abused, but that would be a tricky refactor (and would require typing
extensions until py3.7 is...