NEP29: Set minimum required version to NumPy 1.20+

[maxrjones]

Description of proposed changes

Following NEP29 policy where NumPy 1.19 is to be dropped on or after June. 21, 2022 (in a minor version increment, i.e. for PyGMT v0.7.0). Bumps minimum supported NumPy version to 1.20 in the setup.py, requirements.txt and environment.yml files. Also update installation documentation and set CI tests to run on NumPy 1.20.

xref #1675

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 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]

Ok, as long as the tests pass, just need to update the required tests to use Python 3.8 / NumPy 1.20. Thanks Max!