Support pytest=8

[manics]

pytest 8 breaks our tests:

ERROR tests/base/node - pytest.PytestRemovedIn8Warning: The (fspath: py.path.local) argument to LocalRepo is deprecated. Please use the (path: pathlib.Path) argument instead.
See https://docs.pytest.org/en/latest/deprecations.html#fspath-argument-for-node-constructors-replaced-with-pathlib-path

Reported in #1329 (comment)

[minrk]

preemptively blocking pytest 9? Are there known planned incompatibilities to deal with?

[manics]

Nothing known, I'm just fed up of dealing with random breakages! Do you want me to remove the upper limit?

[yuvipanda]

Ya, I think we should remove the limit as otherwise we may get stuck here. In general I think all upper limit pins should come with a time expiry.

Otherwise happy to merge this.

[yuvipanda]

The test failure is real but unrelated:

1: package ‘IRkernel’ is not available (for R version 3.6.3)

So I'm going to merge this.