indicate supported dependency versions #87

Currently the project uses a setup.py approach where supported versions of dependencies are not indicated. This can result in unexpected breakage (for example, the current version of python-bitcointx does not work with latest secp256k1 v0.7.0 (#88 , #83)).

Suggestion: Migrate to pyproject.yaml

• How to handle libsecp256k1 ABI non-guarantees of stability #59