Fix typos and add codespell pre-commit hook

[kianmeng]

Typos found via codespell -S .mypy_cache

Note that I've encountered this while running pre-commit:

mypy.....................................................................Failed
- hook id: mypy
- exit code: 1

src/magicgui/widgets/_docs_sigs.py:28: error: Item "None" of "Optional[Any]"
has no attribute "__origin__"  [union-attr]
                        if valtype.__bound__.__origin__ is Union:
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/magicgui/widgets/_docs_sigs.py:30: error: Item "None" of "Optional[Any]"
has no attribute "__args__"  [union-attr]
    ...                       [str(c.__name__) for c in valtype.__bound__.__a...
                                                        ^~~~~~~~~~~~~~~~~~~~~...
Found 2 errors in 1 file (checked 42 source files)

[tlambert03]

Thanks @kianmeng

[kianmeng]

🥳 🥳 🥳 🥳 🥳