Add note in CONTRIBUTING.md about ways to run the mypy type checker.

[ShadowJonathan]

Per @clokep's comment, maybe adding something along the following lines to CONTRIBUTING.md would be nice:

• Something about being able to use tox -e mypy to do a isolated lint check.

• Something about being able to do pip install -e .[mypy] to install the right mypy version in venv, to be able to execute mypy by itself (which can be faster than running it through tox)

[devanshushah1]

Per @clokep's comment, maybe adding something along the following lines to CONTRIBUTING.md would be nice:

• Something about being able to use tox -e mypy to do a isolated lint check.
• Something about being able to do pip install -e .[mypy] to install the right mypy version in venv, to be able to execute mypy by itself (which can be faster than running it through tox)

i agree