changed pyproject to allow python 3.8 or higher, updated tests workfl… #2

Job	Run time
Build MDANSE and run tests on ubuntu-20.04	1m 44s
Build MDANSE and run tests on ubuntu-20.04	2m 11s
Build MDANSE and run tests on ubuntu-20.04	2m 4s
Build MDANSE and run tests on ubuntu-20.04	2m 2s
Build MDANSE and run tests on windows-2019	3m 44s
Build MDANSE and run tests on windows-2019	4m 20s
Build MDANSE and run tests on windows-2019	2m 39s
Build MDANSE and run tests on windows-2019	3m 38s
Build MDANSE and run tests on macOS-11	3m 1s
Build MDANSE and run tests on macOS-11	3m 47s
Build MDANSE and run tests on macOS-11	3m 40s
Build MDANSE and run tests on macOS-11	4m 36s
	37m 26s

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