Create skip_if_no helper function to skip tests when missing a package

[weiji14]

Description of proposed changes

Add a helper testing function which returns a pytest.mark.skipif mark that will skip certain unit tests when a package is not installed. This will be useful for skipping unit tests that require optional dependencies such as pyarrow (#2800).

This is adapted from pandas' internal skip_if_no function at https://github.com/pandas-dev/pandas/blob/v2.1.4/pandas/util/_test_decorators.py#L121. Added a unit test to ensure that a correct pytest mark is returned.

Addresses #2774 (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.
• Use underscores (not hyphens) in names of Python files and directories.

Slash Commands

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• /format: automatically format and lint the code
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