upgrade to Py37+

[fametrano]

With Python2 being already unsupported (even if erroneously reported in pypy as supported) I took a chance at upgradng to py37+ standard.

If this is well received, next step might be to annotate the code (mypy) and the libary would be up to the best current python coding standars

This PR is built on top of https://github.com/rustyrussell/secp256k1-py/pull/12

[jnicoulaud-ledger]

Any chance to merge this ? :pray: