CI: fix python 3.6 not available in ubuntu-latest

[mariobuikhuizen]

Unfinished business of #100

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d16f038) 50.00% compared to head (c92af5f) 50.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #104 +/- ## ======================================= Coverage 50.00% 50.00% ======================================= Files 7 7 Lines 140 140 ======================================= Hits 70 70 Misses 70 70 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=glue-viz). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=glue-viz)

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[maartenbreddels]

Hmm, too bad, because the idea did work...

[maartenbreddels]

lets remove windows from the matrix, i think it's good enough we have 1 3.6 run.