TST: Centralize file downloads #4051

Job	Run time
Build & verify package	18s
pytest on 3.7 (crypto-lib: cryptography)	3m 37s
Check code style issues	29s
pytest on 3.8 (crypto-lib: cryptography)	4m 6s
pytest on windows	4m 48s
pytest on 3.9 (crypto-lib: cryptography)	3m 59s
pytest on 3.10 (crypto-lib: cryptography)	3m 37s
pytest on 3.11 (crypto-lib: cryptography)	3m 35s
pytest on 3.12 (crypto-lib: cryptography)	4m 35s
pytest on 3.6 (crypto-lib: pycryptodome)	3m 40s
pytest on 3.7 (crypto-lib: none)	3m 51s
Combine & check coverage.	12s
	36m 47s

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