Add __eq__ to HashAlgorithm and padding instances

[mathiasertl]

This PR:

1. Adds __eq__() to all subclasses of HashAlgorithm.
2. Adds __eq__() to all padding classes.

My primary use case is testing. First, it's really un-pythonic to me (obviously very subjective, but it feels just not right to me) to write assert isinstance(cert.cert.signature_hash_algorithm, hashes.SHA256()), when I could write assert cert.cert.signature_hash_algorithm == hashes.SHA256(). But once you start comparing to other data structures, it gets even uglier:

def test_something() -> None:
    ...
    assert isinstance(cert.signature_hash_algorithm, type(other.signature_hash_algorithm))

I think this just looks a lot better and readable:

def test_something() -> None:
    ...
    assert cert.signature_hash_algorithm == other.signature_hash_algorithm

[reaperhulk]

Let's put these in a separate PR. Whether or not we want to do __eq__ is possibly debateable, but if you want these properties to be public that's a separate discussion.

[mathiasertl]

Hi @reaperhulk

Thanks for the feedback - I've updated this PR to only include the __eq__ instances. I'll make a separate PR for the extra properties.

kr, Mat