5: Add 2.x related warnings r=ltratt a=nanjekyejoannah

I have broken away the warning bit from the [flag](python#3 ) and the [port ](python#4 )PR. Well, the way function calls are done between C and Python is confusing, nothing scary anyway, review maybe a bit annoying.

Review this PR before python#4 

Co-authored-by: Joannah Nanjekye <jnanjekye@python.org>

7: Port cmp with no extra slot r=ltratt a=nanjekyejoannah

Due to segfaults introducing a new `tp_compare` slot proved problematic. I have found a way of supporting `cmp` without a new slot. Tests are updated to match the new functionality where Py2.x doesn't fail.

I wanted to force push on [this branch] (https://github.com/softdevteam/pygrate3) but maybe you wanted to compare before I force push.

This replaces python#4 



Co-authored-by: Joannah Nanjekye <jnanjekye@python.org>