Add `YearMonth`, `MonthDay` #206

Job	Run time
Test pure Python version (3.9)	17s
Linting	41s
Test pure Python version (3.10)	20s
Test Python 3.9	29s
Test on ubuntu-latest	43s
Test pure Python version (3.11)	20s
Test Python 3.10	28s
Test on windows-latest	2m 47s
Test pure Python version (3.12)	37s
Typecheck	43s
Test Python 3.11	46s
Test on macos-latest	33s
Test pure Python version (3.13)	26s
Test Python 3.12	34s
Test pure Python version (pypy3.9)	30s
Test Python 3.13	40s
Test pure Python version (pypy3.10)	31s
	11m 25s

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