Adjust for pandas 1.5.0

[khaeru]

pandas v1.5.0 was released 2022-09-19. Among other changes:

• The tests TestCachingBackend::test_del_ts and test_del_ts fail on most CI jobs.
• There are numerous warnings like:

  /home/runner/work/ixmp/ixmp/ixmp/backend/jdbc.py:995: FutureWarning: In a future version, `df.iloc[:, i] = newvals` will attempt to set the values inplace instead of always setting a new array. To retain the old behavior, use either `df[df.columns[i]] = newvals` or, if columns are non-unique, `df.isetitem(i, newvals)`
    result.loc[:, "value"] = item.getValues(jList)[:]

• Type stubs are now supplied, so mypy exclusion can be removed: https://github.com/iiasa/ixmp/blob/a5a64d282b691190615558ed590f0897ac4aa959/pyproject.toml#L95
• Possibly others.

Since the only test failures are in caching, ixmp may otherwise work with pandas 1.5.0. Any other symptoms can be collected in this issue.

[khaeru]

• Type stubs are now supplied, so mypy exclusion can be removed:

It turns out these are not yet mature:

The stubs are likely incomplete in terms of covering the published API of pandas.

…so this can wait until they are.