Support user defined generic field types in generic models for python < 3.9.

[daviskirk]

Add support for user defined generic type fields in generic models for python < 3.9.

Referencing this comment (and the problem described) here: #2465 (comment)

Change Summary

The actual change (the check for type_.name_) is not necessary in python >= 3.9 as described in the code comment but is needed for python < 3.9.

Related issue number

Work on #2465 (see comments)

Checklist

• Unit tests for the changes exist
• Tests pass on CI and coverage remains at 100%
• Documentation reflects the changes where applicable
• changes/<pull request or issue id>-<github username>.md file added describing change
  (see changes/README.md for details)

[codecov]

Codecov Report

Merging #2554 (661c3b3) into master (7b7e705) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 661c3b3 differs from pull request most recent head 6c95fe1. Consider uploading reports for the commit 6c95fe1 to get more accurate results

@@            Coverage Diff            @@
##            master     #2554   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           25        25           
  Lines         5109      5109           
  Branches      1050      1050           
=========================================
  Hits          5109      5109           

Impacted Files	Coverage Δ
pydantic/generics.py	100.00% <100.00%> (ø)

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[daviskirk]

Rebased on master to fix tests

[samuelcolvin]

thanks so much.