CI: Pin to sphinx>=6.2 for docs build on Python 3.13

[weiji14]

Description of proposed changes

Fix ModuleNotFoundError: No module named 'imghdr' on Python 3.13. Xref sphinx-doc/sphinx#10440 (comment)

Full traceback from https://readthedocs.org/projects/pygmt-dev/builds/26462203/ in #3490:

sphinx-autogen -i -t _templates -o api/generated api/*.rst
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygmt-dev/conda/3490/bin/sphinx-autogen", line 6, in <module>
    from sphinx.ext.autosummary.generate import main
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygmt-dev/conda/3490/lib/python3.13/site-packages/sphinx/ext/autosummary/__init__.py", line 88, in <module>
    from sphinx.writers.html import HTMLTranslator
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygmt-dev/conda/3490/lib/python3.13/site-packages/sphinx/writers/html.py", line 21, in <module>
    from sphinx.util.images import get_image_size
  File "/home/docs/checkouts/readthedocs.org/user_builds/pygmt-dev/conda/3490/lib/python3.13/site-packages/sphinx/util/images.py", line 4, in <module>
    import imghdr
ModuleNotFoundError: No module named 'imghdr'
make: *** [Makefile:29: api] Error 1

Helps with #3490

Preview:

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.

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[seisman]

I'm a little surprise to see that the CI is still using sphinx 5.x even though the latest version is sphinx 8.x.

[seisman]

We also need to update the needs_sphinx key in doc/conf.py.

pygmt/doc/conf.py

Line 174 in 7845e9e

needs_sphinx = "1.8"