Removes the unsupported "ON CONFLICT DO NOTHING" clause

[ArthurNRL]

Subject:

Feature or Bugfix

• Bugfix

Purpose

• This PR marks supports_ignore_conflicts as an unsupported database feature and overrides default postgres backend function so it doesn't use the unsupported "ON CONFLICT DO NOTHING" clause.

Detail

• Feature added
• Postgres docs

Relates

• Issue

[codecov]

Codecov Report

Merging #103 (b158c6b) into master (57b3d4a) will increase coverage by 0.69%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   64.00%   64.70%   +0.69%     
==========================================
  Files           4        4              
  Lines         439      442       +3     
  Branches      127      127              
==========================================
+ Hits          281      286       +5     
+ Misses        116      115       -1     
+ Partials       42       41       -1     

Impacted Files	Coverage Δ
django_redshift_backend/base.py	63.99% <100.00%> (+0.75%)	⬆️

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[shimizukawa]

Thank you for your cooperation!
I have two requests.

1. please fix the Lint error I am getting in CI.
   https://github.com/jazzband/django-redshift-backend/runs/6991793050?check_suite_focus=true
   pre-commit.ci also has an error, but it is fine as is!

2. can you also write a UnitTest?
   I think there is a UnitTest at https://github.com/jazzband/django-redshift-backend/blob/master/tests/test_redshift_backend.py that you can refer to.

[ArthurNRL]

I can do that this weekend

[ArthurNRL]

Hey, I fixed the lint issue, but I'm having trouble setting the test case up. I would need to run migrations on a db since model.add() needs the model to be saved on the db, but none of the tests in there, other than the migration ones, actually interact with the db.