Update CI, add codespell and flake8

[cclauss]

What does this implement/fix? Explain your changes.

• Update CI to Python 3.8 on Linux, macOS, and Window
• Unify requirements files, sort requirements to make it easier to spot duplicates when requirements grow.
• Add codespell and flake8
• Fix Travis CI build config issues

Does this close any currently open issues?

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Any relevant logs, error output, etc?

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Any other comments?

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Where has this been tested?

Operating System: ...

Platform: ...

Target Platform: ...

[gnebbia]

I am trying to test your PR, but it seems that Travis CI fails when doing pytest, since it does not find any tests.

I have this in the logs: ============================ no tests ran in 0.16s =============================

Coverage.py warning: No data was collected. (no-data-collected)

The command "pytest" exited with 5.

Done. Your build exited with 1.

Any idea about what could cause this?

[cclauss]

??? https://travis-ci.com/github/gnebbia/kb/jobs/389002876#L288 Is your branch current?

[gnebbia]

Merged the travis config! Thanks a lot Sir! Very useful!