make eval_type_backport an optional dependency #33

Job	Run time
Ruff linting, formating, MyPy	41s
Test with coverage (3.8, ubuntu-latest)	3m 41s
Windows/MacOS unit tests (3.8, macos-latest)	59s
Test with coverage (3.9, ubuntu-latest)	3m 34s
Windows/MacOS unit tests (3.8, windows-latest)	1m 36s
Test with coverage (3.10, ubuntu-latest)	3m 37s
Windows/MacOS unit tests (3.9, macos-latest)	33s
Test with coverage (3.11, ubuntu-latest)	3m 31s
Test with coverage (3.12, ubuntu-latest)	3m 42s
Windows/MacOS unit tests (3.9, windows-latest)	1m 27s
Windows/MacOS unit tests (3.10, macos-latest)	32s
Windows/MacOS unit tests (3.10, windows-latest)	1m 31s
Windows/MacOS unit tests (3.11, macos-latest)	49s
Windows/MacOS unit tests (3.11, windows-latest)	1m 31s
Windows/MacOS unit tests (3.12, macos-latest)	31s
Windows/MacOS unit tests (3.12, windows-latest)	1m 35s
	29m 50s

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