Bip32

- Support Coincurve up to version 18. This version includes support for BIP340 x-only keys and
Schnorr signatures.

- Implement a pure Python fallback for RIPEMD160 in case `hashlib` does not provide it.
NOTE: the Python implementation was copied from a test-only implementation and is not constant
time. This shouldn't be an issue for the usage we make of it here.

- Actually support Coincurve up to version 17..

This is a breaking release.

- Python 3.10 support.
- Drop Python 3.6 support (EOL).
- Support Coincurve up to version 17.

- Two new methods were added: `get_xpub_bytes()` and `get_xpriv_bytes()`

This is a breaking release.

- Added test vector 5 for more sanity checks when deserializing an xpub or an
xpriv (see https://github.com/bitcoin/bips/pull/921).
- Renamed `get_master_xpub` and `get_master_xpriv` to `get_xpub` and `get_xpriv`.
- We now refuse to create a `BIP32` object with:
- An unknown network
- A depth of 0 (master) and a non-0 index or fingerprint