CI refactoring

[antonag32]

This pull request tackles two points:

• read the docs support: Adds config, changes conf.py to be compatible with new build flows
• removes dead code from CI platforms that are no longer in use
• splits tox environments into logical units per runner
• drops python3.5 and python3.6 from pipeline.

Closes #205.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.34% :warning:

Comparison is base (9cd4715) 81.98% compared to head (f086fbe) 81.64%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #206 +/- ## ========================================== - Coverage 81.98% 81.64% -0.34% ========================================== Files 7 7 Lines 594 594 Branches 111 108 -3 ========================================== - Hits 487 485 -2 - Misses 77 79 +2 Partials 30 30 ``` [see 1 file with indirect coverage changes](https://app.codecov.io/gh/Vauxoo/travis2docker/pull/206/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Vauxoo)

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