Release 9.0.0

[github-actions]

Created by the prepare release pr workflow.

Once all builds pass and it has been approved by one or more maintainers, start the deploy workflow, using these parameters:

• Use workflow from: release-9.0.0.
• Release version: 9.0.0.

Or execute on the command line:

gh workflow run deploy.yml -r release-9.0.0 -f version=9.0.0

After the workflow has been approved by a core maintainer, the package will be uploaded to PyPI automatically.

[bluetech]

I will edit the changes later, but any comments welcome in the meantime.

The RTD build has some error in subtests.rst:

/home/docs/checkouts/readthedocs.org/user_builds/pytest/checkouts/13878/doc/en/how-to/subtests.rst:29: WARNING: Lexing literal_block '$ pytest -q test_subtest.py\nuuuuuF                                                               [100%]\n================================= FAILURES =================================\n_______________________ test [custom message] (i=1) ________________________\n\nsubtests = <_pytest.subtests.Subtests object at 0xdeadbeef0001>\n\n    def test(subtests):\n        for i in range(5):\n            with subtests.test(msg="custom message", i=i):\n>               assert i % 2 == 0\nE               assert (1 % 2) == 0\n\ntest_subtest.py:6: AssertionError\n_______________________ test [custom message] (i=3) ________________________\n\nsubtests = <_pytest.subtests.Subtests object at 0xdeadbeef0001>\n\n    def test(subtests):\n        for i in range(5):\n            with subtests.test(msg="custom message", i=i):\n>               assert i % 2 == 0\nE               assert (3 % 2) == 0\n\ntest_subtest.py:6: AssertionError\n___________________________________ test ___________________________________\ncontains 2 failed subtests\n========================= short test summary info ==========================\nSUBFAILED[custom message] (i=1) test_subtest.py::test - assert (1 % 2) == 0\nSUBFAILED[custom message] (i=3) test_subtest.py::test - assert (3 % 2) == 0\nFAILED test_subtest.py::test - contains 2 failed subtests\n3 failed, 3 subtests passed in 0.12s' as "pytest" resulted in an error at token: 'u'. Retrying in relaxed mode. [misc.highlighting_failure]