BUG: Update pyproject.toml with minimum Python version of 3.8. #5346

Job	Run time
pytest on windows	7m 45s
Check code style issues	41s
Build & verify package	24s
pytest on 3.8 (crypto-lib: cryptography)	5m 37s
pytest on 3.9 (crypto-lib: cryptography)	5m 58s
pytest on 3.10 (crypto-lib: cryptography)	5m 54s
pytest on 3.11 (crypto-lib: cryptography)	5m 24s
pytest on 3.12 (crypto-lib: cryptography)	7m 54s
pytest on 3.13-dev (crypto-lib: cryptography)	7m 51s
pytest on 3.8 (crypto-lib: pycryptodome)	6m 17s
pytest on 3.8 (crypto-lib: none)	5m 59s
Combine & check coverage.	17s
	1h 0m 1s

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