Chore: close some debts after kafka-python merge

[ods]

Changes

• Drop isinstance hack (not needed anymore)
• Add 2.6 case to check_version

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (72c1969) 95.07% compared to head (de6942b) 95.08%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #962   +/-   ##
=======================================
  Coverage   95.07%   95.08%           
=======================================
  Files         106      106           
  Lines       16323    16323           
  Branches     2610     2610           
=======================================
+ Hits        15519    15520    +1     
  Misses        530      530           
+ Partials      274      273    -1     

Flag	Coverage Δ
cext	91.76% <100.00%> (-0.01%)	⬇️
integration	94.72% <100.00%> (+<0.01%)	⬆️
purepy	94.57% <100.00%> (+0.02%)	⬆️
unit	44.85% <66.66%> (+0.01%)	⬆️

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

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