Add unit testing with GitHub actions (linux, macos and windows)

[alexamici]

After Travis-CI has changed its terms of service several Open Source projects are moving to GitHub Actions.

This PR adds tests with GitHub actions and remove Travis-CI.

Note that the code-style workflows fails due to a few trivial black failures. They can be fixed in a different PR. (fixed by installing the version of black in conda-forge, not defaults)

Windows write unit tests fail mostly due to a problem with the template folder.

[codecov-io]

Codecov Report

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The diff coverage is n/a.

@@            Coverage Diff             @@
##             develop      #42   +/-   ##
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  Coverage           ?   77.11%           
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  Files              ?       20           
  Lines              ?     2229           
  Branches           ?      200           
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  Hits               ?     1719           
  Misses             ?      453           
  Partials           ?       57           

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[shahramn]

Dear Alessandro,
I would like to keep the Travis stuff for now. Can you re-submit your PR without the travis yaml deletion please?

[shahramn]

Dear Alessandro,
I would like to keep the Travis stuff for now. Can you re-submit your PR without the travis yaml deletion please

[alexamici]

@shahramn apparently I don't have write permission to the repo anymore, so I can't update the files in branch and thus the PR.

This appears to be a recent change as I did create the branch and the PR by pushing to the eccodes-python repo.

I can fork and create another PR later in the week.

[shahramn]

I have merged this to a separate branch which will later be merged to develop