schemaview.py: add type_roots method

[ialarmedalien]

Add a type_roots function, analogous to existing class_roots and slot_roots functions, to find all base types in a schema.

N.b. I haven't added a type_leaves function as I don't have any need of it, but it would be easy enough to do so.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.37%. Comparing base (1b67264) to head (3b6fcde).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #468      +/-   ##
==========================================
+ Coverage   77.36%   77.37%   +0.01%     
==========================================
  Files          52       52              
  Lines        4470     4473       +3     
  Branches      975      975              
==========================================
+ Hits         3458     3461       +3     
  Misses        785      785              
  Partials      227      227              

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