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.
Add setuptools' find_packages() to avoid repeated updates to the list of packages.
Add setup() arguments, such as classifiers (Trove classifiers used for project classification and searching), keywords, and extra requirements.
Add pip requirements installation file.
Update supported tuf installation options:
Installing from pip and installing an extras requirement (pip install tuf[fast_ed25519]).
