Support Python 3.10

[weiji14]

Description of proposed changes

Python 3.10 has been released on 7 October 2021, changelog is at https://docs.python.org/3.10/whatsnew/3.10.html. See previous PR for Python 3.9 at #689.

This PR is part 1/2 for adding Python 3.10 support to PyGMT, see also the sister PR at #1577 which will update the ci_test.yaml and ci_tests_dev.yaml continuous integration tests.

Note that this PR updates the publish-to-pypi.yml and style-checks.yml GitHub Actions workflows have been updated to use Python 3.10 in this PR. These two work because it uses the setup-python instead of setup-miniconda.

Fixes #

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 docstrings or tutorials.

Slash Commands

You can write slash commands (/command) in the first line of a comment to perform
specific operations. Supported slash commands are:

• /format: automatically format and lint the code
• /test-gmt-dev: run full tests on the latest GMT development version

[weiji14]

Ok, I've checked that make package (building the PyGMT source and wheel distribution) works on Python 3.10 via the publish-to-pypi.yml workflow (see https://github.com/GenericMappingTools/pygmt/runs/4028836891?check_suite_focus=true#step:7:1275), so people using Python 3.10 should be able to install PyGMT from PyPI 🤞