Add a table for compatibility of PyGMT with Python and GMT

[seisman]

Description of proposed changes

Fixes #748.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

Notes

• You can write /format in the first line of a comment to lint the code automatically

[seisman]

We can add the 0.3.0 entry now or later.

[weiji14]

Let's add it later, not too sure yet when we want to cut a 0.3.0 release?

[seisman]

I think we can release v0.3.0 two weeks before GMT v6.2.0 (late Jan or early Feb).

[weiji14]

Since we went no-arch in v0.2.1 (see #690), should the Python compatibility be >=3.6? Not sure how to test on Python 3.10 but it should technically work.

[seisman]

Do Python 3.6 codes always work with future Python versions?

[weiji14]

It should? I think async/await became reserved names in 3.7 but PyGMT doesn't use those, or any advanced stuff really. Things should be safe until Python 4 at least.

[seisman]

OK, changed to ">=3.6".