Add fallback for ripemd160 when not available in hashlib

[Eunovo]

Resolves #30

[Eunovo]

@darosior I got this error while trying to run tests
ImportError while importing test module '.../python-bip380/tests/test_miniscript.py'. Hint: make sure your test modules/packages have valid Python names. Traceback: /usr/lib/python3.10/importlib/__init__.py:126: in import_module return _bootstrap._gcd_import(name[level:], package, level) tests/test_miniscript.py:7: in <module> from bitcointx.core import ( ../../.local/lib/python3.10/site-packages/bitcointx/core/__init__.py:25: in <module> from . import script ../../.local/lib/python3.10/site-packages/bitcointx/core/script.py:31: in <module> import bitcointx.core.key ../../.local/lib/python3.10/site-packages/bitcointx/core/key.py:39: in <module> from bitcointx.core.secp256k1 import ( ../../.local/lib/python3.10/site-packages/bitcointx/core/secp256k1.py:250: in <module> _secp256k1 = load_secp256k1_library(bitcointx.util._secp256k1_library_path) ../../.local/lib/python3.10/site-packages/bitcointx/core/secp256k1.py:235: in load_secp256k1_library raise ImportError('secp256k1 library not found') E ImportError: secp256k1 library not found

[jp1ac4]

I think this needs to be a relative path, i.e. .ripemd_fallback