Replace OpTree with OP_TREE

[daxfohl]

There was a comment in op_tree.py to do this once mypy supports recursive types. The referenced issue python/mypy#731 is now closed, and the suggested replacement seems to work.

cc @maffoo

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.88%. Comparing base (4206cb1) to head (ac0c202).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6960      +/-   ##
==========================================
- Coverage   97.88%   97.88%   -0.01%     
==========================================
  Files        1085     1085              
  Lines       94962    94957       -5     
==========================================
- Hits        92952    92947       -5     
  Misses       2010     2010              

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