Add `YearMonth`, `MonthDay` #204

Job	Run time
Linting	40s
Test Python 3.9	31s
Test on ubuntu-latest	41s
Test pure Python version (3.9)	17s
Test Python 3.10	31s
Test on windows-latest	1m 54s
Typecheck	23s
Test pure Python version (3.10)	15s
Test on macos-latest	58s
Test pure Python version (3.11)	14s
Test Python 3.11	31s
Test pure Python version (3.12)	34s
Test pure Python version (3.13)	28s
Test pure Python version (pypy3.9)	34s
Test Python 3.12	40s
Test pure Python version (pypy3.10)	32s
Test Python 3.13	42s
	10m 25s

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