Accept mmap objects in places where bytearray is expected.

[jonathanslenders]

Fixes #1467.

[srittau]

I assume that mmap actually implements the buffer protocol, but I didn't check. We should preferably fix python/typing#593.

[JelleZijlstra]

Seems reasonable, even if typing.Buffer is a better solution

[JelleZijlstra]

mypy is unhappy with this because of various incompatible method overrides. I suppose we either have to # type: ignore most of the class or implement typing.Buffer.

[srittau]

I am going to close this for now. Please reopen, if you want to try fixing the build errors.