Use the ReadableBuffer type in more places

[bmerry]

This is a follow-up on #4232. memoryview, hashlib, and hmac are updated
to use ReadableBuffer type instead of their own home-spun unions of
bytes, bytearray and whatever else each use case used. mmap is being
handled in #4244, and I'll leave BinaryIO for another day (or possibly
another person) because it's going to require some messy code
duplication because the relevant methods are defined in IO[AnyStr].

There's one corner case I'm not quite sure how best to handle: the
documentation for hmac.digest claim that the parmaeters have the same
meanings as in hmac.new, but in CPython the latter has an explicit check
that key is bytes or bytearray while the former works with a
memory-view. For now I've matched the documentation.

Also, the documentation for HMAC.update says that msg can be any type
supported by hashlib from Python 3.4; but I can't see anything in the
Python 2.7 implementation that would prevent it also taking bytes-like
objects, so I've not tried to treat Python 2 any different to Python 3.

[JelleZijlstra]

Looks good, except that you need to apply the change to the other copy of the builtins module (https://travis-ci.org/github/python/typeshed/jobs/700063485).

[bmerry]

Looks good, except that you need to apply the change to the other copy of the builtins module

Thanks, I hadn't been aware of that. Apologies for not checking the Travis status myself.

[bmerry]

@JelleZijlstra looks like this time it's passed CI.