Support NumPy 2.1 (only)

Going forward you likely want to support NumPy 2.0 (some breaking changes), and I think users can choose it already. 2.1 is out.

Since you support NumPy expliclity, do some of the changes concern you, such as:

>np.float32(3) + 3. now returns a float32 when it previously returned a float64.

No files, tests, or entry points are named. Start by locating PythonCall.jl's explicit NumPy support and checking the NumPy 2.0 and 2.1 compatibility changes, including the float32 addition behavior described here. Done means the project supports NumPy 2.1 as intended and the affected behavior is covered by verification.

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