gh-135797: Add _field_types to abstract AST classes - #159567

Classes without fields have always had an empty _fields tuple, but they never got a _field_types dictionary, so the attribute was missing on ast.AST and on the abstract classes such as ast.mod, while typeshed declares it on ast.AST. make_type() now sets an empty dictionary on every class it creates, and ast.AST gets one too

the constructor used the missing attribute to tell user subclasses that declare _fields but no _field_types apart from the built-in nodes. an empty dictionary is handled the same way now, so those subclasses keep the behavior they have had before 3.13; without that, class Fields(ast.AST): _fields = ('a',) raises TypeError: Field 'a' is missing from Fields._field_types.

Lib/test/test_ast/test_ast.py gets a test checking that every class in the ast module has a _field_types dictionary whose keys match _fields, and test_fields_but_no_field_types covers the subclass behavior above. test_ast passes, also with -R 3:3

• Issue: ast.class._field_types causes Exception when ast.class._fields does not cause Exception #135797