Small incompatibility with tox 4.1.2

[frenzymadness]

See: https://github.com/tox-dev/tox/commit/a3d3ec042d38195392841a9112911c2bde3587d1

tox now runs pip freeze if running in CI is detected. The problem is that our dummy installer here does not have installed method.

We can either:

• add a dummy method returning nothing because the list of installed packages is not that important in our use cases, or
• add a simple method inspired from https://github.com/tox-dev/tox/blob/43382af3a228c4dfc098ec50e136971553ee0035/src/tox/tox_env/python/pip/pip_install.py#L69 or
• make our dummy installer inherit from the official one. IMHO if we'd have this it'd work because pip freeze should work for us but there is a risk that something changes upstream which breaks our expectations and it might lead to our plugin installing packages outside venv or something so we'd have to be very careful about that.

I'm not sure what is the best way forward in regard to the maintenance burden.

There is only one package in Fedora where this is an issue: https://src.fedoraproject.org/rpms/python-tornado/blob/rawhide/f/python-tornado.spec#_58

[hroncok]

See also https://github.com/fedora-python/tox-current-env/pull/32