Restrict the units that can be added to PlainYearMonth #2100

Job	Run time
Test feature matrix	12m 19s
Lint	3m 56s
Documentation	27s
MSRV	23s
Test (stable, ubuntu-latest)	32s
Test (stable, windows-latest)	2m 5s
Test FFI (stable, ubuntu-latest)	30s
Test (stable, macos-latest)	26s
	20m 38s

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