models: make sure uid is compared case-sensitive

[nijel]

Proposed changes

The database collation might vary, so use safer comparing in Python.

Types of changes

Please check the type of change your PR introduces:

• Release (new release request)
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (PEP8, lint, formatting, renaming, etc)
• Refactoring (no functional changes, no api changes)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build related changes (build process, tests runner, etc)
• Other (please describe):

Checklist

Put an x in the boxes that apply. You can also fill these out after creating
the PR. If you're unsure about any of them, don't hesitate to ask. We're here to
help! This is simply a reminder of what we are going to look for before merging
your code.

• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added documentation to https://github.com/python-social-auth/social-docs

Other information

Any other information that is important to this PR such as screenshots of how
the component looks before and after the change.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.63%. Comparing base (fb93814) to head (7028c70).
Report is 15 commits behind head on master.

Files	Patch %	Lines
social_django/models.py	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
- Coverage   91.72%   91.63%   -0.09%     
==========================================
  Files          39       39              
  Lines        1172     1172              
  Branches      144      146       +2     
==========================================
- Hits         1075     1074       -1     
  Misses         72       72              
- Partials       25       26       +1     

Flag	Coverage Δ
unittests	91.63% <80.00%> (-0.09%)	⬇️

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.

[sevdog]

This is a known issue with MySQL and also stated in django docs: https://docs.djangoproject.com/en/stable/ref/databases/#collation-settings (at least from Django 1.8).