[BUG] Pypy3 & mypy do not work together

[niccokunzmann]

Describe the bug

We install mypy generally for all tests.
However, installation under pypy3 does not work with mypy==1.19.0.

To reproduce

pypy3 -m pip install mypy

inside of

tox -e pypy3

Expected behavior

We should be able to run the tox environment tests.

Environment

• Operating system: Ubuntu 24
• Python version: pypy3
• icalendar version: 7.0.0a2

Additional context

• Mypy 1.19.0 can't be installed on pypy python/mypy#20329
• Release v7.0.0a2 #1012
• e2bac2a