add p2wpkh-p2sh (ypub), p2wpkh (zpub) support

[sh4dowb]

I needed to create ypub and zpub extended keys, so I made this small modification.
note: if this doesn't get accepted, for future readers, you can use this instead:
base58.b58encode_check(base58.b58decode_check(xpub_here).replace(0x0488B21E.to_bytes(4, "big"), 0x04b24746.to_bytes(4, "big"))).decode()

[darosior]

Thanks for the pull request.

I think the hardcoding of specific script types inside the encoding of an extended public key is an unscalable and terrible layer violation that we need to stop disseminate. This is the reason why this library intentionally does not provide support for this (aiming to only implement the BIP32 specification).

That said, given how widespread this practice is i can hear that users of the library may need the conversion no matter what i think the right thing is (eg to work with legacy applications).
To this end i'd be much more comfortable with a pair of standalone functions doing the xpub<->whatever-pub conversion similarly to what is described in the OP, with a note in the doc comment that it should not be used in favour of output descriptors (see also here).