Add tomli as a dependency in GMT Dev Tests

[weiji14]

Description of proposed changes

Adding tomli to fix the GMT Dev Tests breakages reported in #1392. This is a temporary measure until pytest-cov releases a new version > 2.12.1 to incorporate the dependency fix in pytest-dev/pytest-cov#410.

While looking into #1392, I noticed that the pip install --pre ... command in ci_tests_dev.yaml doesn't seem to produce any output in the GitHub Actions log (see e.g. https://github.com/GenericMappingTools/pygmt/runs/3206665246?check_suite_focus=true#step:7:4). So are the pip packages actually being installed? Not sure if this is a bug introduced after #1105, so am investigating here. Edit: ok, using pip list at 0a8ab3d shows that the pip packages are indeed installed.

Fixes #1392

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.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[seisman]

Looks good to me. I think we should merge it before releasing v0.4.1.