feat: replace python-dateutil with stdlib datetime.fromisoformat #3334

Job	Run time
Integration Tests (pdm.minimal.lock)	25s
test_min_deps (macos-latest)	1m 4s
test_min_deps (ubuntu-latest)	47s
Integration Tests (pdm.lock)	24s
test_min_deps (windows-latest)	1m 21s
test (3.11, macos-latest)	30s
test (3.12, windows-latest)	1m 36s
test (3.10, ubuntu-latest)	1m 1s
test (3.10, macos-latest)	50s
test (3.11, ubuntu-latest)	56s
test (3.13, ubuntu-latest)	53s
test (3.10, windows-latest)	1m 23s
test (3.11, windows-latest)	1m 25s
test (3.14, ubuntu-latest)	49s
test (3.12, macos-latest)	52s
test (3.14, macos-latest)	41s
test (3.12, ubuntu-latest)	54s
test (3.13, windows-latest)	1m 25s
test (3.13, macos-latest)	34s
test (3.14, windows-latest)	1m 18s
Combine & check coverage	12s
	19m 20s

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