Re-organization of test and packaging infrastructure for dbfread

[zaneselvans]

• Switched over to using Tox to coordinate tests, and a src/ directory with normal non-relative imports of packages.
• Removed anything pertaining to Python 2 compatibility.
• Switched to using setuptools exclusively, rather than disttools
• Updated package metadata to reflect current version compatibility and to mention Catalyst.
• dbf2sqlite is now a script that's deployed by setup.py
• Set up GitHub actions to run Tox tests and Sphinx Tox builds
• Consolidated all tests under the tests/ directory to avoid pytest modifying the sys.path and making everything in the src directory importable.
• Updated version to 2.1.1 to differentiate from previous version in git+https:// install for PUDL.

[codecov[bot]]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@802d0ef). Click here to learn what that means. The diff coverage is n/a.

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  Lines             ?      489           
  Branches          ?        0           
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  Hits              ?      399           
  Misses            ?       90           
  Partials          ?        0           

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