HDWallet.from_index missing return statement

[kyurais]

The Issue:

Deriving from index returns None if hardened is set to True in all cases

>>> from hdwallet import HDWallet
>>> from hdwallet.symbols import BTC
>>> hdwallet = HDWallet(symbol=BTC)
>>> hdwallet.from_xprivate_key(xprivate_key="xprv9s21ZrQH143K3xPGUzpogJeKtRdjHkK6muBJo8v7rEVRzT83xJgNcLpMoJXUf9wJFKfuHR4SGvfgdShh4t9VmjjrE9usBunK3LfNna31LGF")
>>> child_node = hdwallet.from_index(index=44, hardened=True)
>>> child_node is None
True

Expected Behavior:

>>> child_node = hdwallet.from_index(index=44, hardened=True)
<hdwallet.hdwallet.HDWallet object at 0x000001E8BFB98D60>
>>> isinstance(child_node, HDWallet)
True

Added return statement to HDWallet.from_index hardened key derivation.
Change return type to Optional to mirror the return type of called method HDWallet._derive_key_by_index

Do let me know if the pull request is not formulated correctly.

[kyurais]

Duplicate #85