Continue refactoring the test cases of test_updater.py.
Fix issue where repository_tool.py was not writing new compressed metadata.
Minor edits to TUF modules.
Support the following installation setup:
# Minimal install intended for clients (pure Python, only ed25519 signatures).
pip install tuf
# Optional installation required by the TUF repository tools (RSA keys and rsassa-pss
# signatures, faster ed25519 cryptographic computations, general-purpose cryptography, etc.)
pip install tuf[tools]
Re-implement the fix for issue #167.
Update comments and docstrings related to the fixes above.
Update modules affected by the changes made to the latest versions of pyca-ed25519 and pyca-pynacl:
Do not use the unsafe key and signature generation functions of pure python ed25519, but do support the signature verification routine. Developers must use the faster and secure pynacl+libsodium to generate ed25519 keys and signatures.
Temporarily suppress pynacl's import warning error.
Minor edits to comments and code.
Update the key modules and their unit tests so that only the signature scheme is included in the signature label.
PyCrypto-PKCS#1 PSS -> RSASSA-PSS
ed25519-python and ed25519-pynacl -> ed25519
