If a caller calls a `list()` method with both `iterator=True` (or
`as_list=False`) and `page=X` then emit a `UserWarning` as the options
are mutually exclusive.

iterator=True/as_list=False are used

This fix a regression introduced on commit
https://github.com/python-gitlab/python-gitlab/commit/1339d645ce58a2e1198b898b9549ba5917b1ff12

If page is used, then get_next should be false.

This was found on the mesa ci project, after upgrading the python-gitlab
version, the script that monitors the ci was getting killed by consuming
too much memory.

tox > 4 no longer will re-use the tox directory :(  What this means is
that with the previous config if you ran:
    $ tox -e mypy; tox -e isort; tox -e mypy
It would recreate the tox environment each time :(

By removing the `envdir` values it will have the tox environments in
separate directories and not recreate them.

The have an FAQ entry about this:
https://tox.wiki/en/latest/upgrading.html#re-use-of-environments