Add daily unit tests

[BenBarrettQC]

Closes #429.

I've set this up for Linux so far (pending your feedback I will add macOS and Windows) and issues are opened on CI failure (see e.g. #499).

Some additional comments:

• Some tests against the nightly builds of pandas, numpy and scikit-learn are currently running into errors due to this, which gets imported upstream here.
• Other tests against the nightly builds are failing here because the option context is finding duplicate keys; I suspect we just need to add e.g. "display." to max_rows etc.
• The other daily tests are currently passing (i.e. non-nightly tests), see e.g. here.
• Once you're happy with the PR I'll comment in the code section that opens issues on CI failure; I wanted to avoid spamming everyone for now.
• Test against tabmat nightlies #498 to follow.

Checklist

• Added a CHANGELOG.rst entry

[BenBarrettQC]

Minor update:

1. I've now bundled Test against tabmat nightlies #498 into this PR.
2. Based on preliminary discussions with @jtilly it seems only running tests against nightly builds on Linux might be sufficient for now.

[jtilly]

Cool, thanks! Almost ready to merge.

• Can you please open a separate issue in glum for the dask_ml.preprocessing.DummyEncoder problem and then don't test against the scikit-learn nightlies until that's fixed?
• Can you fix the display max_rows issue? (You can do this as part of this PR.)
• Can you remove the commented out code from the other workflows?