Remove system_packages from readthedocs config.

[mkelley]

This is a mandatory change since 2023 Sep 25: https://blog.readthedocs.com/migrate-configuration-v2/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (36285d0) 76.84% compared to head (a92bda3) 76.84%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #391 +/- ## ======================================= Coverage 76.84% 76.84% ======================================= Files 78 78 Lines 6974 6974 ======================================= Hits 5359 5359 Misses 1615 1615 ```

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