remove mypy_extensions dep #684

Job	Run time
linters (3.9, ubuntu-latest)	2m 7s
Testing on Python 3.8 (ubuntu-latest)	2m 44s
Testing on Python 3.8 (macOS-latest)	4m 44s
Testing on Python 3.8 (windows-latest)	6m 17s
Testing on Python 3.9 (ubuntu-latest)	2m 30s
Testing on Python 3.9 (macOS-latest)	3m 38s
Testing on Python 3.9 (windows-latest)	7m 46s
Testing on Python 3.10 (ubuntu-latest)	2m 28s
Testing on Python 3.10 (macOS-latest)	3m 36s
Testing on Python 3.10 (windows-latest)	4m 43s
Testing on Python 3.11 (ubuntu-latest)	2m 25s
Testing on Python 3.11 (macOS-latest)	4m 30s
Testing on Python 3.11 (windows-latest)	6m 41s
	54m 9s

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