Add note about asyncnullcontext

[Cito]

• Mention in the docstring of asyncnullcontext that it is only needed for old Python versions (but we need to keep it in as long as we support Python 3.9)
• Test that asyncnullcontext works exactly like nullcontext for newer Python versions
• Update dependencies and adapt tests to latest pytest_httpx

[coveralls]

Pull Request Test Coverage Report for Build 11935378528

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 90.83%

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Files with Coverage Reduction	New Missed Lines	%
.tox/py312/lib/python3.12/site-packages/ghga_service_commons/api/mock_router.py	1	96.58%

Totals
Change from base Build 11723443134:	-0.1%
Covered Lines:	733
Relevant Lines:	807

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💛 - Coveralls