CI: Start testing on Python 3.11

[EwoutH]

Add a Python 3.11 job (on Ubuntu) to the testing matrix, since the first Python 3.11 release candidate is out.

[coveralls]

Coverage remained the same at 81.139% when pulling ff21348 on CI-Py311 into f040e03 on master.

[EwoutH]

Not all dependencies currently support Python 3.11. Either waiting until they all do or reducing the number of (blocking) dependencies (see #130) will solve this.

So draft for now.

[EwoutH]

Retriggered the CI, currently fails on SciPy since they don't have Python 3.11 wheels available on PyPI yet.

[EwoutH]

@quaquel statsmodels now builds on Python 3.11, and I was done waiting on a release. So I modified the dependencies in the pyproject.toml that for Python 3.11, it installs statsmodels from their default branch. This works in CI, but also on regular users installs. So this means that pip install ema_workbench should now work on Python 3.11, albeit taking a bit more time (because statsmodels is build from source.

For Python 3.10 and lower, nothing has changed.

Special thanks to @jwodder for helping me out with the toml syntax in pypa/setuptools#3651!

(Please don't squash when merging, the second commit needs to be reversed when a new statsmodels version is released. So we're going to do a fast-forward merge on this one.)

[quaquel]

Looks good to me. We do indeed need to update this once statsmodels is readily available for 3.11