Set minimum required version of dependencies

[leouieda]

Establish the minimum supported version of run-time and optional dependencies (see fatiando/community#40). Support all releases from the past 2 years with a minimum of 2 minor versions.

Needed to do some maintenance work on our CI and packaging setup to enable this:

1. Moved the requirements into separate files in the env folder.
2. Moved package metadata to setup.cfg and pyproject.toml so that we can programmatically convert run-time dependencies into a requirements.txt file with tools/export_requirements.py
3. Made a tools/oldest_requirements.py that converts >= into ==. Could have been a sed call but getting this to work on Mac and Linux was a huge pain. The Python script is easier.
4. Simplify the CI scripts to use the requirements files and tools scripts to build their list of packages.

I also ended up moving license_notice.py to tools since we have more of them now.

Almost everything worked perfectly with the oldest versions out of the box. The only modification I had to make was removing some of the xxhash algorithms from the tests since they were introduced in a later version. This shouldn't be a problem because the underlying implementation is the same either way.

Also addresses fatiando/community#42

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 and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• 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.

[leouieda]

@dopplershift this is largely based on our twitter chat the other day and the MetPy configuration. Would you be able to give this a quick look? No rush on this, whenever you have a spare moment.

[dopplershift]

Looks good!

And to be clear, users don't even need to care about the requirements.txt thing--unless it causes installation problems due to overly tight pinning. It's something I run into while reviewing conda-forge recipes.

[dopplershift]

If you have a version when these should start passing, instead of removing them you could wrap them with skipif--assuming you could detect what version you have.

[leouieda]

👍🏽 good idea! Implementing that now.

[leouieda]

Thanks for the feedback, Ryan! It was time we modernized our build system anyway and we've forgotten to package a requirement.txt file in the past so this is good to help avoid that in the future.