I know I did that, so I'm owning my mistake, and rename "make quality"
by "make lint". A linter is something that checks your code for errors,
and both flake8 and mypy are linters.

There are now 2 new make commands:

* make lint-style: perform any code style check (today: flake8 only),
  it could be used for a pylint check for example, or isort, or rst
  check, or anything that could help us with code style in the future
* make lint-type: perform any type check (today: mypy only)

The command `quality` is replaced by the command `lint`, which runs both
`lint-style` and `lint-type`.

Type check is now mandatory in CI, and the PR template has been updated
accordingly.