Add info about SDK python version policy in compatibility policy docs

[approxit]

Regarding upgrading yapapi minimal python version, we discussed and decided to drop support from python 3.6 and 3.7, and make 3.8 as minimal python version. Side product of this discussion was a realization to mention python version policy somewhere in the docs.

We decided that SDK libraries will support only python versions that are at least receiving security updates. See table of python version with support dates.

Todo:

• [ ] Add information about our approach to python version in compatibility policy docs.