Change setup.py development status flag to stable

[leouieda]

We left it flagged as "alpha" but Pooch has been stable and production
ready for a while now.

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 adding new functionality, add an example to the docstring, gallery, and/or tutorials.