Support Python 3.6+

[janbridley]

Legacy support is a nice-to-have for a lightweight package like this.

Test configuration

To avoid wasted CI time, I have split the test runs into two groups. Modern python versions (3.9+) are run first on Ubuntu-latest. If these pass, a second batch is run on legacy versions 3.6-3.8. The code to run tests is consolidated into a workflow callable which takes in the python version and runner os and generates a job for each combination of input matrixes. Once the package is open source, we can test on Windows (modern and legacy) plus macOS arm (modern).

Can we go lower?

Maybe, but not easily. The tests are based off of Gemmi, which is not available below python 3.6. In addition, f-strings were introduced in python 3.6.The CI does not build, as the pypi link was unable to be found. This could be made to work, but we're getting into the realm of changes that make the experience worse on modern versions for very little benefit. If anyone has a real use case for this I will revisit the question.

Motivation and Context

Resolves #10

Types of Changes

• Documentation update
• Bug fix
• New feature

Checklist:

• I am familiar with the Development Guidelines
• The changes introduced by this pull request are covered by existing or newly introduced tests.
• I have updated the changelog and added my name to the credits.

[joaander]

Thanks!

I have one suggestion to improve reproducibility of the CI builds, but that doesn't block Python 3.6 support.

[joaander]

To make CI tests more reproducible, you should pin specific versions of packages in requirements.in, then use pip-compile to generate requirements.txt that pins all transitive dependencies. That way, an update to a transitive dependency is less likely to cause CI failures from one build to the next with no changes to parsnip. Readthedocs has a nice write up: https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#pin-your-transitive-dependencies

Dependabot knows how to update requirements.in and rerun pip-compile. Add pip to the dependabot config and you will get PRs that test the updated pins: https://github.com/glotzerlab/gsd/blob/trunk-patch/.github/dependabot.yml