Remove Python 3.8 support

[ulgens]

Checklist

• I have read the OpenWISP Contributing Guidelines.
• I have manually tested the changes proposed in this pull request.
• I have written new test cases for new code and/or updated existing tests for changes to existing code.
• I have updated the documentation.

Reference to Existing Issue

Closes #314 .

Description of Changes

• Removes the majority of the compatibility code for <3.9 versions of Python
• Remove 3.8 from CI and docs
• Modernize the code for Python 3.9 with the help of pyupgrade.

[ulgens]

One thing I couldn't find enough time to handle is GeoJsonDict. There is

remove this when support for python 2.6/2.7 will be dropped

note in the dosctring, but it's not clear what it refers to. It's been a while since 2.6/2.7 got EOL, and it looks like the class has multiple relatively complex usages in the codebase. I can use some help.

[nemesifier]

@ulgens thx 🙏.
The build is failing due to a QA check, an unused import.

[ulgens]

@nemesifier Should be okay now

[ulgens]

It fails because of the commit messages now. I feel like the QA is a bit strict, not sure how much value is provided by having [action] prefix on every commit but I can update them if needed.

[nemesifier]

It fails because of the commit messages now. I feel like the QA is a bit strict, not sure how much value is provided by having [action] prefix on every commit but I can update them if needed.

It helps the change log, but don't worry too much about it if you can't get it, we can sort it out.