Remove Python 2 compatibility shims

[Harmon758]

This PR removes and replaces Python 2 compatibility shims, since Python 2 support was dropped with c880f6b in 3.0.0:

• Remove compat.memoryview and compat.PY3
• Remove and replace compat.buffer and compat.to_bytes
• Remove and replace compat.izip with zip, compat.MAXSIZE with sys.maxsize, compat.xrange with range
• Remove and replace encoding.string_types and encoding.text_type with str
• Remove and replace izip in TestPack

[Byron]

Absolutely fantastic, thanks so much for your work!
I will be releasing this in a moment, and publish a new version of GitPython as well with adjusted requirements.