Move matplotlib import inside the check_figures_equal function

[seisman]

Description of proposed changes

Move matplotlib import statement inside the check_figures_equal function, so that the cache_data workflow doesn't need matplotlib installed.

Address #1823 (comment)

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 wrapping a new module, open a 'Wrap new GMT module' issue and submit reasonably-sized PRs.
• 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]

Now the cache_data works well without matplotlib (https://github.com/GenericMappingTools/pygmt/runs/5971739015?check_suite_focus=true).