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Here are some notes in case I wear out before finding a fix.
 analyze_name() gets to the SET_SCOPE(dict, name,
GLOBAL_IMPLICIT) line because the name does not have the
DEF_LOCAL flag set as it should.  It does not have DEF_LOCAL
because Name.ctx for 'g' is Load.  I believe there should be
a set_context() call in ast_for_expr_stmt, as flagged as
TODO by  Jeremy.  Maybe set_context(..., Store, ...) would
work or maybe things need to be changed to allow ctx to have
AugAssign as a value.