add readthedocs.yaml, drop codecov, update python versions, fix doc building

[flokli]

• readthedocs will require a .readthedocs.yaml file - check https://blog.readthedocs.com/migrate-configuration-v2/ for details.
• update python versions
• fix doc building

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (e696759) 46.93% compared to head (e813832) 46.93%.

:exclamation: Current head e813832 differs from pull request most recent head f0a7539. Consider uploading reports for the commit f0a7539 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #333 +/- ## ======================================= Coverage 46.93% 46.93% ======================================= Files 30 30 Lines 1078 1078 ======================================= Hits 506 506 Misses 572 572 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `46.93% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=afewmail#carryforward-flags-in-the-pull-request-comment) to find out more.

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