Remove python 3.8 compatibility

[d33bs]

Description

This PR removes Python 3.8 compatibility from the project. Python 3.8 is now end-of-life as of this month. Removing this constraint on the project will help to alleviate dependency issues, such as errors experienced in #240 .

Closes #171

What is the nature of your change?

• Bug fix (fixes an issue).
• Enhancement (adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• This change requires a documentation update.

Checklist

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• I have read the CONTRIBUTING.md guidelines.
• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• New and existing unit tests pass locally with my changes.
• I have added tests that prove my fix is effective or that my feature works.
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[d33bs]

Thanks @gwaybio ! Merging this in.