chore: fix type annotations

[nijel]

Properly annotate strategy and deal with the mess.

[codecov]

Codecov Report

❌ Patch coverage is 75.71429% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.98%. Comparing base (06921a6) to head (f927986).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/openshift.py	0.00%	3 Missing ⚠️
social_core/backends/odnoklassniki.py	0.00%	2 Missing ⚠️
social_core/backends/slack.py	33.33%	1 Missing and 1 partial ⚠️
social_core/backends/trello.py	0.00%	2 Missing ⚠️
social_core/backends/legacy.py	85.71%	1 Missing ⚠️
social_core/backends/linkedin.py	50.00%	0 Missing and 1 partial ⚠️
social_core/backends/mediawiki.py	0.00%	1 Missing ⚠️
social_core/backends/open_id.py	75.00%	1 Missing ⚠️
social_core/backends/open_id_connect.py	50.00%	1 Missing ⚠️
social_core/backends/ping.py	0.00%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1501      +/-   ##
==========================================
- Coverage   78.99%   78.98%   -0.01%     
==========================================
  Files         354      354              
  Lines       11511    11516       +5     
  Branches      530      530              
==========================================
+ Hits         9093     9096       +3     
- Misses       2236     2238       +2     
  Partials      182      182              

Flag	Coverage Δ
unittests	78.98% <75.71%> (-0.01%)	⬇️

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