bump version to v16.3

[barbieri]

fixes:

• Don't close user-provided requests sessions (@reconman)
• codegen: fix boolean_value with Visitor pattern

updated the tooling:

• moved from setuptools to poetry
• use pre-commit.com instead of custom git hooks
• use black code formatter
• use pytest instead of nosetest

[coveralls]

Pull Request Test Coverage Report for Build 5338446524

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 5338389022:	0.0%
Covered Lines:	1606
Relevant Lines:	1606

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💛 - Coveralls