Add `YearMonth`, `MonthDay` #210

Job	Run time
Linting	12m 22s
Test pure Python version (3.9)	20s
Test Python 3.9	34s
Test pure Python version (3.10)	17s
Test on ubuntu-latest	10m 41s
Typecheck	34s
Test Python 3.10	10m 35s
Test pure Python version (3.11)	18s
Test on windows-latest	1m 43s
Test Python 3.11	31s
Test pure Python version (3.12)	10m 37s
Test on macos-latest	36s
Test Python 3.12	33s
Test pure Python version (3.13)	10m 28s
Test Python 3.13	48s
Test pure Python version (pypy3.9)	5m 31s
Test pure Python version (pypy3.10)	36s
	1h 7m 4s

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