DOC: use black to fix code style in doc pandas-dev#36777
Partially addresses #36777. 

Fixed the following files:
- `doc/source/development/extending.rst`
- `doc/source/user_guide/duplicates.rst`
- `doc/source/user_guide/gotchas.rst`
- `doc/source/user_guide/scale.rst`
Thanks @PrayagS  @dsaxton Could you please add the label `hacktoberfest-accepted` if you find the PR suitable for Hacktoberfest 2020?