MNT:(deps): Bump sphinx from 2.4.4 to 3.1.2 in /ci
MNT:(deps): Bump sphinx from 2.4.4 to 3.1.2 in /ci

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.4.4 to 3.1.2.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/3.x/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v2.4.4...v3.1.2)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@github.com>
Unidata/MetPy,ShawnMurd/MetPy,dopplershift/MetPy,Unidata/MetPy,dopplershift/MetPy