Upgrade six package to fix issues with new pip

[bjester]

Summary

When setting up a Kolibri dev instance with pip>=20, pip becomes broken after running pip install -r requirements/base.txt --upgrade. This is because that version of pip requires a higher version of six than the pinned version Kolibri has specified in base.txt

$ pip install -r requirements/dev.txt --upgrade
Traceback (most recent call last):
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/bin/pip", line 8, in <module>
    sys.exit(main())
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/cli/main.py", line 73, in main
    command = create_command(cmd_name, isolated=("--isolated" in cmd_args))
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/commands/__init__.py", line 96, in create_command
    module = importlib.import_module(module_path)
  File "/usr/lib/python3.6/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 994, in _gcd_import
  File "<frozen importlib._bootstrap>", line 971, in _find_and_load
  File "<frozen importlib._bootstrap>", line 955, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 665, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 24, in <module>
    from pip._internal.cli.req_command import RequirementCommand
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/cli/req_command.py", line 20, in <module>
    from pip._internal.operations.prepare import RequirementPreparer
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/operations/prepare.py", line 16, in <module>
    from pip._internal.distributions import (
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/distributions/__init__.py", line 2, in <module>
    from pip._internal.distributions.wheel import WheelDistribution
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/distributions/wheel.py", line 5, in <module>
    from pip._internal.utils.wheel import pkg_resources_distribution_for_wheel
  File "/home/bjester/.local/share/virtualenvs/kolibri-J0Tqdocd/lib/python3.6/site-packages/pip/_internal/utils/wheel.py", line 12, in <module>
    from pip._vendor.six import PY2, ensure_str
ImportError: cannot import name 'ensure_str'

References

pypa/virtualenv#1551

---

Contributor Checklist

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• PR has the correct target branch and milestone
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• Contributor has fully tested the PR manually
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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

[rtibbles]

If travis is ok with it, I'm ok with it.

[codecov]

Codecov Report

Merging #6871 into develop will not change coverage.
The diff coverage is n/a.