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[bot]]

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

```diff @@ Coverage Diff @@ ## master #416 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 22 22 Lines 2069 2069 Branches 334 334 ========================================= Hits 2069 2069 ```

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