Add support for Python 3.9

[hugovk]

(First step before adding 3.10: https://github.com/fatiando/boule/issues/85.)

Reminders:

• [x] Run make format and make check to make sure the code follows the style guide.
• [n/a] Add tests for new features or tests that would have caught the bug that you're fixing.
• [n/a] Add new public functions/methods/classes to doc/api/index.rst and the base __init__.py file for the package.
• [n/a] Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• [n/a] If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• [x] Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[welcome[bot]]

💖 Thanks for opening your first pull request! 💖

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A few things to keep in mind:

• Remember to run make format to make sure your code follows our style guide.
• If you need help writing tests, take a look at the existing ones for inspiration. If you don't know where to start, let us know and we'll walk you through it.
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