Pin specific package versions in the various requirements*.txt files

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Use a pyproject.toml to specify generic dependencies and replace setup.* files.

Use pip-tools pip-compile to generate requirements.txt for run-time dependencies, requirements-test.txt for test dependencies, and requirements-dev.txt for other development tools and support. This permits using pip-sync in TOX to synchronize tox environments with the dependencies.

Pinning specific versions will avoid the "problem" of a new release of Flake-8 flagging previously acceptable code as now unacceptable because of a more stringent check. With a pinned version, these changes will be limited to being part of planned dependency upgrades.

The version numbers can be upgraded by running pip-compile --upgrade.