Black lint to remove unneeded commas

[weiji14]

Description of proposed changes

Yet another Style Check fix. Black v20.8b0 (released on 2020-08-27) (and newer) treats trailing commas as a signal to split the code onto a newline. E.g.:

somefunction(A="abc",)

would reformat to:

somefunction(
    A="abc",
)

And this causes the Style Checks test to fail (see https://github.com/GenericMappingTools/pygmt/runs/1033691109?check_suite_focus=true). Note that I had to explicitly use pip install --upgrade black to get this new version, as the conda-forge black-feedstock still uses black=19.10b0.

See also:

• https://github.com/psf/black/blob/master/CHANGES.md#208b0
• Finish "magic trailing comma" handling psf/black#1288

Fixes #

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.