Remove Python 2.7 support

[rtibbles]

Summary

• Removes Python 2.7 support
• Removes all testing on Python 2.7
• Updates dist build pipeline to build in lowest supported Python version - 3.6
• Updates PEX build to build in Python 3.6
• Cleans up conditional code paths for Python 2.7
• Removes futures imports
• Removes vendored ipaddress module
• Removes use of six module
• Removes unneeded dependencies

References

Fixes #1963
Fixes #10363
Fixes #10484

Reviewer guidance

Do all tests still pass? Do the WHL and PEX file still work?

For review, I would recommend going commit by commit - each one handles a different logical section of the removal. The futures imports removals are rote, but widespread, the six removals a bit more constrained but hit a lot of files.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b10.dev0_git.357.g67f990dc.pex
Windows Installer (EXE)	kolibri-0.16.0b10.dev0+git.357.g67f990dc-unsigned.exe
Debian Package	kolibri_0.16.0b10.dev0+git.357.g67f990dc-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b10.dev0+git.357.g67f990dc-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b10.dev0+git.357.g67f990dc-0.1.0-debug.apk
TAR file	kolibri-0.16.0b10.dev0+git.357.g67f990dc.tar.gz
WHL file	kolibri-0.16.0b10.dev0+git.357.g67f990dc-py2.py3-none-any.whl

[bjester]

A freshly groomed Kolibri!