Make libsecp256k1 a mandatory dependency, drop the fallback pure-python code that was only used when running from sources without libsecp256k1 compiled.
Hard fail is there is any import error or unavailable symbol in secp256k1 (lib compiled without a required module, lib from Bitcoin Core...)
This is a partial backport of [[https://github.com/spesmilo/electrum/pull/5947 | electrum#5947]]
https://github.com/spesmilo/electrum/pull/5947/commits/ad408ea832c55458b730945832c826894dfa9386
Depends on D16687