Fix docs/requirements.txt

[PekkaSavolainen]

This PR makes docs/requirements.txt compatible with Spine Toolbox requirements. When merged this commit should be cherry picked to master as well.

Re spine-tools/Spine-Toolbox#2342

Checklist before merging

• [ ] Documentation (also in Toolbox repo) is up-to-date
• [ ] Release notes in Toolbox repo have been updated
• [ ] Unit tests have been added/updated accordingly
• [ ] Code has been formatted by black
• [x] Unit tests pass

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (af34079) 76.36% compared to head (12e6bb8) 76.36%.

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 0.8-dev #278 +/- ## ======================================== Coverage 76.36% 76.36% ======================================== Files 76 76 Lines 8996 8996 Branches 1888 1888 ======================================== Hits 6870 6870 Misses 1793 1793 Partials 333 333 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.