Add poetry check to CI

[johnthagen]

It turns out that poetry lock --check only validates a subset of what poetry check validates (such as Trove classifiers).

This would be improved if the following upstream issue were completed:

• Replace poetry lock --check with poetry check --lock python-poetry/poetry#6756

Should add poetry check in addition to poetry lock --check to CI.

This is blocked however by this upstream Poetry issue:

• Trove classifiers prefixed with "Private ::" fail poetry check python-poetry/poetry#7167

[johnthagen]

• Support Private :: trove classifiers python-poetry/poetry#7271

was merged, so next Poetry release this should be possible to fix.

[johnthagen]

Poetry 1.4.0 is released, but there seems to be some issues with its installer dependency

• https://github.com/python-poetry/poetry/issues?q=is%3Aissue+is%3Aopen+1.4.0

Should wait on this before recommending Poetry 1.4.x.