Add support for Multi-phase extension module initialization PEP489

[rcmcdonald91]

• Adds support for PEP489.
• Indirectly addresses a missing Py_INCREF that can cause segfaults when used by embedded interpreters.

[rcmcdonald91]

My first attempt at python extension work, I am addressing failing tests.

In the interim, I have created another PR #106 that directly addresses the missing Py_INCREF issue. #105 needs more work.

[rcmcdonald91]

This likely requires a broader discussion on the minimum python version we want to support.

[oschwald]

We generally support all Python versions that are still getting security updates upstream. That would mean that we will want to support 3.7 until June.

I haven't looked too closely at the code yet, but it would be fine to conditionally compile blocks of code for particular Python versions.