Explains why pytato was being added as a requirement for downstream
packages.

- add pytato to requirements.txt of downstream projects to make them
  compatible with PytatoArrayContext

Co-authored-by: Andreas Klöckner <inform@tiker.net>

- fixes function types, thereby passing mypy CI

* add run-mypy.sh

* use for github CI

* add to gitlab

* mypy -> python -m mypy

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* add run-pylint.sh

* add to gitignore