exclude py36-djdev, py37-djdev. use newest python version

[shimizukawa]

Feature or Bugfix

• Bugfix

Purpose

• exclude djdev-py36 and djdev-py37 testing because djdev (django-4.0) drop support py36 and py37.

Relates

• https://code.djangoproject.com/ticket/32355

[codecov]

Codecov Report

Merging #76 (35d31e3) into master (a1f543a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   45.49%   45.49%           
=======================================
  Files           3        3           
  Lines         244      244           
  Branches       62       62           
=======================================
  Hits          111      111           
  Misses        123      123           
  Partials       10       10           

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[shimizukawa]

local tox testing also succeeded (without py36).