Makefile: drop python2 support

[jelly]

We should later drop the whole Makefile I guess.

[behrmann]

Change LGTM.

The error is coming from pytest releasing 9.0 yesterday, which no longer supports Python 3.9

[behrmann]

Ok, so the workaround is

[tool.tox.env.mypy]
description = "Run type checking tool mypy"
# pytest-stub required for type hints to be available
-deps = ["mypy", "pytest-stub"]
+deps = ["mypy", "pytest-stub", "pytest<9.0.0"]
commands = [
    ["mypy", "--version"],
    ["mypy", "{posargs:varlink}"]
]

in pyproject.toml.