Add `YearMonth`, `MonthDay` #209

Job	Run time
Linting	33s
Test on ubuntu-latest	10m 40s
Test pure Python version (3.9)	16s
Test on windows-latest	2m 7s
Test Python 3.9	33s
Test pure Python version (3.10)	19s
Typecheck	32s
Test on macos-latest	5m 36s
Test Python 3.10	30s
Test pure Python version (3.11)	20s
Test Python 3.11	28s
Test pure Python version (3.12)	32s
Test Python 3.12	10m 31s
Test pure Python version (3.13)	27s
Test Python 3.13	38s
Test pure Python version (pypy3.9)	32s
Test pure Python version (pypy3.10)	32s
	35m 6s

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