NEP29: Set minimum required version to Python 3.8+

[michaelgrund]

Description of proposed changes

• Remove Python 3.7 from our CI jobs
• Bump the minimum required Python version to 3.8 in the documentation
• Remove Python 3.7 from setup.py

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.

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]

Thanks @michaelgrund! Just some suggestions to put quotes around the version numbers (otherwise Python 3.10 will be read as Python 3.1 in the future, see #1577). I'll reduce the diffs in #1577 a little so that it won't conflict with this PR (edit: done at fca7a68).

[weiji14]

Thanks @michaelgrund, I've changed the PR's title and updated the required checks at https://github.com/GenericMappingTools/pygmt/settings. Just wait for a second approval before merging.