Move pytest import inside of skip_if_no function

[weiji14]

Description of proposed changes

To fix the ModuleNotFoundError: No module named 'pytest' error in the cache_data.yaml GitHub Action that was failing at https://github.com/GenericMappingTools/pygmt/actions/runs/7238500260/job/19719382772#step:5:13

Patches #2883.

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.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

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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