fix: raise minimum Python version for type checking to 3.9

[alexander-held]

The latest version of numpy, 1.25.0, dropped compatibility with Python 3.8. When using mypy on cabinetry with Python 3.8 and the settings from the pyproject.toml, this results in errors getting raised in cabinetry. An example is the following:

import numpy as np

a = np.asarray([1, 2, 3])
b = a.copy()
c = a.copy().astype(float)
d: np.ndarray = a.copy().astype(float)

resulting in

test.py:5: error: Need type annotation for "c"  [var-annotated]
    c = a.copy().astype(float)
        ^~~~~~~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 1 source file)

This is addressed here by raising the minimum Python version for the type checks being performed to Python 3.9.

* raise minimum Python version for mypy to Python 3.9 for compatibility with numpy 1.25.0

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b1c4c40) 100.00% compared to head (fc6a688) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           22        22           
  Lines         2069      2069           
  Branches       334       334           
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  Hits          2069      2069           

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