Remove Python 3.8 support #5884

Job	Run time
lint/style-and-typos	52s
linux/3.9/bare-env	1s
linux/3.9	1s
linux/3.10	1s
linux/3.11	1s
linux/3.12	1s
linux/3.13	1s
osx/3.9	1s
osx/3.10	1s
osx/3.11	1s
osx/3.12	1s
osx/3.13	1s
win/3.9	1s
win/3.10	1s
win/3.11	1s
win/3.12	1s
win/3.13	1s
linux/3.11/conda	1s
linux/3.10/mpi	1s
linux/3.12/cython	1s
win/3.9/pip	1s
linux/3.11/singletest	1s
linux/3.9/slim	1s
linux/3.12/numpy2	1s
linux/3.10/pyutilib	1s
linux/pypy-3.9	1s
process-coverage-${{ matrix.TARGET }}	1s
	1m 18s

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