Remove use of six from the testsuite

[s-t-e-v-e-n-k]

Since Python 3.9 is the lowest version of Python we test with, we can comfortably remove six from the testsuite, and stop installing it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.63%. Comparing base (cfae650) to head (0399e44).
⚠️ Report is 80 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #130       +/-   ##
===========================================
+ Coverage   56.34%   81.63%   +25.28%     
===========================================
  Files          20       38       +18     
  Lines        2765     4682     +1917     
  Branches      397      395        -2     
===========================================
+ Hits         1558     3822     +2264     
+ Misses       1142      793      -349     
- Partials       65       67        +2     

Flag	Coverage Δ
py310-cython-cover	59.92% <100.00%> (?)
py310-pure-cover	67.53% <100.00%> (?)
py311-cython-cover	59.92% <100.00%> (?)
py311-pure-cover	69.37% <100.00%> (?)
py312-pure-cover	69.40% <100.00%> (?)
py313-cython-cover	59.94% <100.00%> (?)
py313-pure-cover	69.40% <100.00%> (?)
py313t-pure-cover	74.21% <100.00%> (?)
py314-cython-cover	59.34% <100.00%> (?)
py314-pure-cover	79.77% <100.00%> (?)
py314t-cython-cover	60.02% <100.00%> (?)
py314t-pure-cover	79.77% <100.00%> (?)
py39-cython-cover	59.86% <100.00%> (?)
py39-pure-cover	69.31% <100.00%> (?)
pypy310-pure-cover	73.87% <100.00%> (?)
pypy39-pure-cover	73.94% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.