This replaces instances of `ecdsa.ecdsa.generator_secp256k1` (instance of `ecdsa.ellipticcurve.Point`) with a `ECPubkey` instance.
After this diff, the only modules still importing `ecdsa` are:
- ecc.py
- ecc_fast.py
- schnorr.py (the pure-python fallback code)
- digitalbitbox.py
The plan is to now make ecc*.py and shnorr.py have a hard dependency on libsecp256k1 and remove the `ecdsa` dependency altogether.
This concludes backport of https://github.com/spesmilo/electrum/commit/16e4827e8c6db833b0a969351b73eb34d489681f
Depends on D16681