Enable black, flake8, check-manifest and pytest

[jwhonce]

• Setup project for Python 3.9+ support, dropping support for 3.6, 3.7 and 3.8.
• tox configured to test supported versions
• Black configured to verify code format
• flake8 configured to verify code sematics
• check-manifest configured to ensure files in project are setup for sdist
• Added simple unit test for ETAPI() constructor and close()
• Code changes to pass verification, mostly line length but some DeprecationWarning's and unbound variable control paths were also refactored.
• Some spelling corrections

To run verify code and run tests:

$ tox

Closes #17

[jwhonce]

@Nriver This became bigger than I like, standing up flake8, black and unit tests cascaded on me.

[Nriver]

Looks good to me.

[jwhonce]

@Nriver Please merge when you are comfortable with the changes. TIA