Fix failing builds and add read the docs configuration

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

[x] 🧪 Added tests for changed code.
[x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
[x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
[x] 📚 Updated documentation for changed code.
[x] ⏩ Code is up-to-date with the master branch.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Fix failing build and drop support for Python 3.6 and 3.7. Update dependencies and add read the docs config.

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (74a759b) 100.00% compared to head (e47fb38) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1163 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 10 10 Lines 2326 2326 Branches 492 400 -92 ========================================= Hits 2326 2326 ``` | [Files Changed](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py) | Coverage Δ | | |---|---|---| | [arrow/factory.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvZmFjdG9yeS5weQ==) | `100.00% <ø> (ø)` | | | [arrow/formatter.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvZm9ybWF0dGVyLnB5) | `100.00% <ø> (ø)` | | | [arrow/arrow.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvYXJyb3cucHk=) | `100.00% <100.00%> (ø)` | | | [arrow/locales.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvbG9jYWxlcy5weQ==) | `100.00% <100.00%> (ø)` | | | [arrow/parser.py](https://app.codecov.io/gh/arrow-py/arrow/pull/1163?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=arrow-py#diff-YXJyb3cvcGFyc2VyLnB5) | `100.00% <100.00%> (ø)` | |

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arrow-py / arrow