migrate from secp256k1 to coincurve

[prusnak]

i think we should just migrate to coincurve - it is well maintained (last secp256k1 release was in 2021) - and even CLN uses coincurve and not secp256k1 for their python plugins

[prusnak]

related darosior/python-bip32#40 - bip32 is stuck on coincurve 19.x, that PR hopefully bumps the dependency to 20.x soon

[callebtc]

Tracking here: darosior/python-bip32#43

[jooray]

Now track here: darosior/python-bip32#47

The new version drops dependencies.