Add test_requirements.txt (fix #377)

[ramnes]

Tox is great. Although, a lot of people are used to the simpler pip install -r test_requirements.txt and pytest workflow, including me.

Adding this test_requirements.txt file doesn't cost much and makes things easier for new developers and for rapid testing.

This PR is related to #377.

[codecov-io]

Codecov Report

Merging #945 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #945   +/-   ##
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  Coverage     100%   100%           
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  Files          17     17           
  Lines         684    684           
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  Hits          684    684

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[hackebrot]

Hi @ramnes! 👋

Thank you for your contribution. 🍪

[hackebrot]

FYI I removed -e . from the requirements file, so we can better use it from tox. Otherwise tox would test against the intermediate distribution.