fixes #92: since django-3.0 sqlmigrate (and migrate) does not work

[shimizukawa]

Feature or Bugfix

• Bugfix

Purpose

• fix manage.py sqlmigrate

Detail

• Django-3.0 changed the implementation of the postgres backend, causing sqlmigrate to stop working.
• https://github.com/django/django/pull/11594/files#diff-e3fea5d09fbc3286aafc3ed3ea77299b7649528754f410afa725045a8d2f74afR165-R184

Relates

• 92

[codecov[bot]]

Codecov Report

Merging #93 (d1f9652) into master (a67bb59) will increase coverage by 0.43%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   47.01%   47.45%   +0.43%     
==========================================
  Files           3        3              
  Lines         251      255       +4     
  Branches       68       68              
==========================================
+ Hits          118      121       +3     
- Misses        123      124       +1     
  Partials       10       10              

Impacted Files	Coverage Δ
django_redshift_backend/base.py	43.93% <66.66%> (+0.52%)	:arrow_up:

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