Use more modern extension module features

[rhpvorderman]

Currently isal_zlibmodule.c has diverged quite a bit from zlibmodule.c.

• A different buffer method
• Using static rather than heap types
• No module isolation

This increases the maintainance burden. Ideally the differences should be minimal. Also this limits the usability of python=isal in a free-threading python world. Good module isolation should be realized before that becomes a reality. Also this allows moving python-isal to the stable ABI at some point in the future.