Small improvements

Added

• Better python3 compatibility
• is_valid() method in BaseAddress to check address validity
• Factory for SegWit scripts, to better handle versioning
• Hash pinning in dependencies

Fixed

• Make some methods private in ExtendedPublicKey

Backward compatibility

Breaks the following old functionalities:

• The serialize_key() method in ExtendedPublicKey has been made private (i.e. renamed to _serialize_key())
• The serialized_public() method in ExtendedPublicKey has been made private (i.e. renamed to _serialized_public())