Various small cleanup changes

[salty-horse]

• Remove unused imports - struct was imported but unused.
• Mention required 'flush' in stream_decompress documentation - the provided class must implement it, but it wasn't documented.
• Switch to HTTPS URLs in documentation
• Fix PyPI capitalization in README
• Remove Python 2 test - version no longer supported
• Enforce minimum Python version - now packaging tools will return an error if the user doesn't have 3.8+
• Link to correct BSD license - the README and the license is the 3-clause "New BSD License", but the link led to the 2-clause BSD license

[martindurant]

Please update actions/upload-artifact in the github actions