Add `python3.8` support

[Danipulok]

Description of the change

Add python 3.8 support. Add tests for python 3.8. All the tests are passing.

Motivation

Some productions (for example, our team) still uses python3.8.

Type of change

Feature request

Checklist

• [x] I have performed a self-review of my code.
• [x] I have added detailed comments to my code where applicable.
• [x] I have verified that my change does not break existing code.
• [x] My PR is based on the latest changes of the main branch (if unsure, please run git pull --rebase upstream main).
• [x] I am familiar with the Google Style Guide for the language I have coded in.
• [x] I have read through the Contributing Guide and signed the Contributor License Agreement.

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[MarkDaoust]

Thanks! but I don't think we're doing this.

Sorry!