requirements: Workaround ImportError when running pytest

markupsafe 2.1.0 removes functions that were used by Jinja2 < 3,
the latter being a dependency of pybadges.

As Jinja2 < 3 is installed in the venv, pinning markupsafe to
a version still compatible with Jinja2 < 3 is the only way to
avoid error when running pytest as some plugins import jinja2.