Remove warnings and fix types.

[hhoppe]

• Fix warning about PIL.Image.Resampling: (Pillow 9.0 deprecates PIL.Image.LANCZOS; requires PIL.Image.Resampling.LANCZOS)
• Fix warning about "dtype == np.bool"; numpy now desires "dtype == bool".
• Fix warnings about use of np.ndarray without subscripting (rather than np.ndarray[Any, Any]).
• Add py.typed to let mypy know this package has type annotations.
• Improve typing of parameters and return values, using numpy types.
• Check code using flake8, mypy, autopep8, and pylint, and tweak code accordingly.
• Replace deprecated np.max() by np.amax().
• Replace random.seed in tests using the more recent numpy random generators.
• In mediapy_test, correct an np.float which should have been np.float64.

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[hedpeter]

Looks good to me! This might hit a merge conflict, as we recently removed the test_image_write function.