Check compatibility with Python 3.8

[seisman]

Description of proposed changes

Test PyGMT with Python 3.8

Fixes #340.

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.

[weiji14]

Cool, it looks like Python 3.8 works! Probably should document somewhere in the README that we support Python 3.6 to 3.8? Just saw that the PyPI badge lists the versions we support, all good.