Support free-threaded Python builds

[weiji14]

Description of the desired feature

Python 3.13 came out with an experimental free-threaded mode with the global-interpreter lock disabled (PEP 703). This issue is to track the addition for PyGMT to build free-threaded compatible wheels.

Main guide: https://py-free-threading.github.io/porting/

Steps:

• Test building PyGMT on Python 3.13t in CI
  • https://py-free-threading.github.io/ci/
• Document thread safety guarantees via trove classifiers
  • https://py-free-threading.github.io/porting/#define-and-document-thread-safety-guarantees
• Validate thread safety using pytest-freethreaded or pytest-run-parallel
  • https://py-free-threading.github.io/testing/

Notes:

• Some GMT modules already support multi-threading via the -x flag, which happens via OpenMP, need to be careful that we don't do threading of many PyGMT functions + call multi-threaded GMT functions
• Related thread on multi-processing support - Support multiprocessing without importing gmt in each process #217
• Dependencies that have added free-threading support
  • NumPy (since 2.1) - https://numpy.org/doc/2.3/reference/thread_safety.html
  • Pandas (since 2.2.3) - https://pandas.pydata.org/pandas-docs/version/2.2/whatsnew/v2.2.3.html#pandas-2-2-3-is-now-compatible-with-python-3-13

Are you willing to help implement and maintain this feature?

Yes