Add required generic type to docutils Writer subclass

[tristanlatr]

Fixes the CI.

[codecov[bot]]

Codecov Report

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

Project coverage is 92.29%. Comparing base (41b734b) to head (d359e15).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #825 +/- ## ======================================= Coverage 92.29% 92.29% ======================================= Files 47 47 Lines 8513 8514 +1 Branches 1695 1695 ======================================= + Hits 7857 7858 +1 Misses 385 385 Partials 271 271 ```

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

[github-actions[bot]]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅

[github-actions[bot]]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅

[github-actions[bot]]

According to pydoctor_primer, this change doesn't affect pydoctor warnings on a corpus of open source code. ✅