Fix PartialMixin.next_step type annotation

[nsoranzo]

Proposed changes

Also:

• Improve annotation of PartialMixin.data
• Remove unused user attribute of UserMixin, which may create type annotation conflicts in derived classes: Discrepancies in column types for social-auth-core derived database tables galaxyproject/galaxy#19617

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.15%. Comparing base (326d4b3) to head (a2e8922).
Report is 87 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1023      +/-   ##
==========================================
+ Coverage   77.88%   78.15%   +0.26%     
==========================================
  Files         347      348       +1     
  Lines       10669    10766      +97     
  Branches      504      457      -47     
==========================================
+ Hits         8310     8414     +104     
+ Misses       2200     2192       -8     
- Partials      159      160       +1     

Flag	Coverage Δ
unittests	78.15% <100.00%> (+0.26%)	⬆️

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.

[nijel]

Merged, thanks for your contribution!