Determine the minimum required versions of dependencies from package metadata for docs

[seisman]

Description of proposed changes

It's possible to determine the minimum required versions from the package metadata using codes like below. So that we don't have to update the doc/minversions.md file everytime we bump the minimum required versions (e.g., #3372).

>>> import importlib
>>> from packaging.requirements import Requirement
>>> requirements = {
...:     Requirement(requirement).name: str(Requirement(requirement).specifier)
...:     for requirement in importlib.metadata.requires("pygmt")
...: }
...: requirements.update(
...:     {
...:         "python": importlib.metadata.metadata("pygmt")["Requires-Python"],
...:         #"gmt": f">={required_gmt_version}",
...:     }
...: )

>> requirements
{'numpy': '>=1.24',
 'pandas': '>=1.5',
 'xarray': '>=2022.09',
 'netCDF4': '',
 'packaging': '',
 'contextily': '',
 'geopandas': '',
 'IPython': '',
 'rioxarray': '',
 'python': '>=3.10'}

This PR uses the same codes to determine the minimum required versions and store in the requirements dictionary, which can be used in the documentation using syntax like {{ requires.python }}.

Preview:

• https://pygmt-dev--3380.org.readthedocs.build/en/3380/minversions.html
• https://pygmt-dev--3380.org.readthedocs.build/en/3380/install.html#which-python

[weiji14]

So much more automation, nice!