Bring back the missing Cython string specification #373

Job	Run time
Build MDANSE and run tests on ubuntu-20.04	4m 43s
Build MDANSE and run tests on ubuntu-20.04	3m 20s
Build MDANSE and run tests on ubuntu-20.04	3m 14s
Build MDANSE and run tests on ubuntu-20.04	3m 52s
Build MDANSE and run tests on windows-2019	7m 10s
Build MDANSE and run tests on windows-2019	5m 38s
Build MDANSE and run tests on windows-2019	6m 8s
Build MDANSE and run tests on windows-2019	4m 46s
Build MDANSE and run tests on macOS-11	6m 1s
Build MDANSE and run tests on macOS-11	6m 14s
Build MDANSE and run tests on macOS-11	6m 29s
Build MDANSE and run tests on macOS-11	7m 8s
	1h 4m 43s

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