chore: stable Python 3.9 support

[KingDarBoja]

GH Actions supports Python 3.9 as stable release. More info: actions/runner-images#1740

[codecov-io]

Codecov Report

Merging #160 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #160   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           14        14           
  Lines          929       929           
=========================================
  Hits           929       929           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c7765c7...84099b8. Read the comment docs.

[leszekhanusz]

LGTM 🚀

Actually the test with 3.9-dev never worked on my machine using tox (with conda) on my machine and this change does not solve this.

I could finally test it by adding:

conda_channels =                                                                                                        
    conda-forge

in the [testenv] section of tox.ini

Maybe we should add this to the CONTRIBUTING.md file

During testing I discovered that there are deprecation warnings under python3.9 and created issue #161

[KingDarBoja]

@leszekhanusz looks like it is still not available on default, but it does on conda-forge or anaconda channels: https://anaconda.org/anaconda/python

Maybe we should add this to the CONTRIBUTING.md file

Yeah, while the Python version is supported by default, I will add it to the tox.ini file in this PR.

During testing I discovered that there are deprecation warnings under python3.9 and created issue

I ignored those while running py3.9-dev locally 😆 but glad you made a PR for that.