Replace setuptools's "platforms" metadata with classifiers

[seisman]

We have the platforms metadata defined in the tool.setuptools section of pyproject.toml. The metadata has been defined since the start of the project (e.g., in v0.1a1: https://github.com/GenericMappingTools/pygmt/blob/2963f624371da4c8858df115b5703a7cc13602ac/setup.py).

pygmt/pyproject.toml

Lines 56 to 58 in 4b9314a

	[tool.setuptools]
	platforms = ["Any"]
	license-files = ["LICENSE.txt"]

If I remember correctly, this metadata was previously shown on PyPI, but it's not shown anymore (see https://pypi.org/project/pygmt, but I can't find any discussions about deprecating this metadata).

Looking at other projects, I think the standard way is to use classifiers instead (xref: https://pypi.org/classifiers/).

• NumPy: https://github.com/numpy/numpy/blob/12364e360a4f389fc372865c6c252f4536540c8c/pyproject.toml#L37-L40
• xarray: https://github.com/pydata/xarray/blob/1622499f9533e268cfca0bd21f3f5a5258530c39/pyproject.toml#L8
• pandas: https://github.com/pandas-dev/pandas/blob/5e50d3f3d2b0ee65f0d5bfda0c6da47ffd39dcfe/pyproject.toml#L40

xarray/pandas use the Operating System :: OS Independent classifier, while numpy uses detailed classifiers like Operating System :: MacOS.

I prefer the numpy style and propose to add the following classifiers (xref: https://pypi.org/classifiers/)

• Operating System :: MacOS
• Operating System :: Microsoft :: Windows
• Operating System :: POSIX
• Operating System :: Unix