Support builtin str removeprefix/removesuffix

[jdimatteo]

These are new in Python 3.9: https://docs.python.org/3/library/stdtypes.html#str.removeprefix

[jdimatteo]

removeprefix and removesuffix are also newly defined on bytes and bytearray in Python 3.9:

https://docs.python.org/3/library/stdtypes.html#bytes.removeprefix

The prefix may be any bytes-like object.

I didn't include these in this PR because I don't know how to represent "bytes-like object" as a type in pytype. It looks like in cpython the type corresponds to PyBytesObject. Would it be sufficient to just use typing.SupportsBytes? I unfortunately don't have a lot of bandwidth to track down these details.

[rchen152]

Sorry for the slow review! This looks good to me; I'll go ahead and import it once the tests finish running, assuming they don't find any unexpected issues.

[rchen152]

re: bytes and bytearray - those are defined as their own classes in builtins.pytd, so you could add the new methods to them in the same way you did for str (just search for class bytes and class bytearray in https://github.com/google/pytype/blob/master/pytype/stubs/builtins/builtins.pytd).