Add function for reading and caching PEP 518 spec #559

Job	Run time
bootstrap / setup	2s
pytest / setup	3s
bootstrap / bootstrap (macos-latest, 3.8)	1m 6s
bootstrap / bootstrap (macos-latest, 3.9)	43s
bootstrap / bootstrap (macos-latest, 3.10)	43s
bootstrap / bootstrap (macos-latest, 3.11)	36s
bootstrap / bootstrap (macos-latest, 3.12)	39s
bootstrap / bootstrap (ubuntu-latest, 3.8)	52s
bootstrap / bootstrap (ubuntu-latest, 3.9)	56s
bootstrap / bootstrap (ubuntu-latest, 3.10)	51s
bootstrap / bootstrap (ubuntu-latest, 3.11)	47s
bootstrap / bootstrap (ubuntu-latest, 3.12)	1m 0s
bootstrap / bootstrap (windows-latest, 3.8)	1m 50s
bootstrap / bootstrap (windows-latest, 3.9)	1m 56s
bootstrap / bootstrap (windows-latest, 3.10)	1m 46s
bootstrap / bootstrap (windows-latest, 3.11)	1m 42s
bootstrap / bootstrap (windows-latest, 3.12)	2m 19s
bootstrap / bootstrap (ubuntu-20.04, 3.6)	53s
pytest / pytest (macos-latest, 3.8)	23s
pytest / pytest (macos-latest, 3.9)	3s
pytest / pytest (macos-latest, 3.10)	39s
pytest / pytest (macos-latest, 3.11)	40s
pytest / pytest (macos-latest, 3.12)	39s
pytest / pytest (ubuntu-latest, 3.8)	27s
pytest / pytest (ubuntu-latest, 3.9)	28s
pytest / pytest (ubuntu-latest, 3.10)	1m 22s
pytest / pytest (ubuntu-latest, 3.11)	1m 6s
pytest / pytest (ubuntu-latest, 3.12)	1m 22s
pytest / pytest (windows-latest, 3.8)	7s
pytest / pytest (windows-latest, 3.9)	7s
pytest / pytest (windows-latest, 3.10)	12s
pytest / pytest (windows-latest, 3.11)	12s
pytest / pytest (windows-latest, 3.12)	1s
pytest / pytest (ubuntu-20.04, 3.6)	1m 4s
	27m 36s

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