[mypyc] Add bytes primitive type

[JukkaL]

This adds minimal support for the bytes primitive type. This won't
improve performance until we implement some more primitives.
Actually, this will likely degrade performance a bit, since we now add
type checks for bytes values.

Unfortunately, most of the bytes operations we'd need are private in
CPython, so we'll need to either depend on implementation details or
reimplement (or copy+paste) a bunch of code.

Work on mypyc/mypyc#644.

[JukkaL]

There were various test failures because of a bad annotation in typed_ast stubs. It should be fixed now.

[JukkaL]

Now we have a problem with group(n) of regular expression objects potentially returning None, but the return type is non-optional in typeshed. I can come up with a workaround here, but we should really have a general fix.

We could fix typeshed -- maybe the return type should be Union[bytes, Any] instead of bytes? In that case mypyc would do the right thing.

In any case these issues are very useful findings. Mypyc is a good at finding typeshed bugs!

[hauntsaninja]

The match group thing keeps coming up, I'm in favour of a Union Any return (with maybe some overloads for when we don't need it): python/typeshed#3902 (comment)

[TH3CHARLie]

What's the status quo of the match group thing? Is it still a blocker of this?

[JukkaL]

I think that it's still an issue. I kind of forgot about this PR. The typeshed fix should be fairly straightforward (but it needs to be synced back to mypy).

[JelleZijlstra]

group was already changed to return AnyStr | Any: https://github.com/python/typeshed/blob/master/stdlib/typing.pyi#L568

[97littleleaf11]

Hi! @hauntsaninja, I'm working on bytes primitives, shall we sync the typeshed to fix current match.group issue?

[hauntsaninja]

Sure, I opened #10862

[JukkaL]

Closing in favor of #10881.