Update coincurve to support python 3.13

[dmytro-samoylenko]

Hello,

Lnurl cannot be installed using Python 3.13 due to coincurve build error. coincurve started to support Python 3.13 in version 21, but lnurl depends on coincurve<21.

Please consider to bump coincurve version to v21.0.0

[dni]

we need this pr to be merged, we depend on bip32 lib. darosior/python-bip32#47