fixed python 3.7 compatibility

[mbway]

in the pyproject.toml file, maturin indicates that the minimum supported python version is 3.7 but the code uses

• subscripted builtins (i.e. list[str] instead of typing.List[str]) which was introduced in python 3.9
• | to denote unions (i.e. str | int instead of typing.Union[str, int]) which was introduced in python 3.10

(the import hook in #1748 also aims to be compatible with python 3.7)

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