OpenFF API and packaging changes have broken the CI

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It appears that unexpected OpenFF related packaging and API changes have broken the CI on all platforms and Python variants. I'll investigate. Hopefully there will be a backwards compatible fix.

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Weirdly, it doesn't look like there have been any version changes. Will go through the shared dependencies to see what could be causing issues. The fact that it fails an all platforms/variants, but with different errors is confusing.

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The issue appears to be ambertools, which is used for testing only. This downgrades the openff-interchange-base version, which breaks things. Not sure why this has happened, since neither ambertools nor any of the openff packages have been updated since the last working CI.

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This is because OpenFF have dropped Python 3.11 support in their 0.18.1 "bugfix" release, i.e. a patch change from 0.18.0. (Currently, this is also completely undocumented in their release notes and requires the commits to be examined manually.) I've resolved this with a conditional pin, but it really shouldn't be necessary.

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This fixes the Python 3.11+ issues, but on Python 3.10 it's now resolving an older version of openff-forcefields, which doesn't include 2.2.1, hence the test failures. I might drop the test to use 2.2.0 instead. What a mess.

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This was closed by #524, although we will no doubt need to revisit in future.