Migrate parse_single_unit to functools.cache #932

Job	Run time
test-linux (3.9)	3m 14s
test-linux (3.9, uncertainties==3.1.6)	3m 44s
test-linux (3.9, uncertainties>=3.1.6,<4.0.0)	4m 46s
test-linux (3.9, numpy>=1.21,<2.0.0)	3m 33s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	4m 0s
test-linux (3.9, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	4m 22s
test-linux (3.10)	4m 45s
test-linux (3.10, uncertainties==3.1.6)	4m 23s
test-linux (3.10, uncertainties>=3.1.6,<4.0.0)	4m 20s
test-linux (3.10, numpy>=1.21,<2.0.0)	6m 48s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	5m 45s
test-linux (3.10, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	5m 45s
test-linux (3.11)	3m 58s
test-linux (3.11, uncertainties==3.1.6)	4m 50s
test-linux (3.11, uncertainties>=3.1.6,<4.0.0)	5m 1s
test-linux (3.11, numpy>=1.21,<2.0.0)	4m 22s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties==3.1.6)	5m 39s
test-linux (3.11, numpy>=1.21,<2.0.0, uncertainties>=3.1.6,<4.0.0)	6m 21s
test-linux (3.9, numpy, matplotlib==2.2.5)	6m 44s
test-linux (3.9, numpy, uncertainties, sparse xarray netCDF4 dask[complete]==2023.4.0 graphviz ba...	5m 4s
test-macos (3.9)	4m 38s
test-macos (3.9, numpy>=1.21,<2.0.0)	4m 51s
test-macos (3.10)	4m 52s
test-macos (3.10, numpy>=1.21,<2.0.0)	6m 29s
test-macos (3.11)	5m 28s
test-macos (3.11, numpy>=1.21,<2.0.0)	5m 46s
test-windows (3.9, numpy>=1.21,<2.0.0)	1m 7s
test-windows (3.10, numpy>=1.21,<2.0.0)	1m 16s
test-windows (3.11, numpy>=1.21,<2.0.0)	57s
	2h 12m 48s

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