Introduce optional dependencies for package

[penguinolog]

Simplify package installation with extras
Note trio and ZeroMQ event loop

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment
• I've included docstrings and/or documentation and/or examples for my code (if this is a new feature)

Fix: #441

[penguinolog]

Note: this PR not introduces normal "requirements"

[coveralls]

Pull Request Test Coverage Report for Build 6348494751

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 74.38%

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Totals
Change from base Build 6341349895:	0.0%
Covered Lines:	7514
Relevant Lines:	10286

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💛 - Coveralls

[wardi]

is there a standard way to get a list of optional dependencies and their descriptions or should we add that here/in the docs?

[ulidtko]

They're described in pyproject.toml... No, don't duplicate in docs, will get outdated

[penguinolog]

Pip feature request is multiple years old, even solution with API is already added to the issue.
At this moment optional dependencies are not visible without API