Three fixes for the accessors generated by Argument Clinic:

• Generating a @getter or a @setter in a preprocessor conditional block failed with KeyError: 'methoddef_name', because the fallback definition was emitted for the METHODDEF symbol instead of the GETSETDEF one.
• The entry of PyGetSetDef is identified by the C basename shared by the accessors, so defining them with different C basenames (Foo.p as foo_get and Foo.p as foo_set), or defining the same accessor twice, silently generated invalid or duplicated entries. It is now an error.
• The setter was called with NULL to delete the attribute. Implementations which did not check for it crashed -- frame.f_trace_opcodes and the context, owner and session attributes of _ssl._SSLSocket -- or reported a confusing SystemError. Deletion is now rejected with AttributeError, unless the new directive @deleter is applied to the setter, which is then called with NULL as before.

@deleter is applied to the 22 setters whose implementation is written to accept NULL, so deleting these attributes works as before. Only three generated files change: the setters of _ssl, _sqlite3.Cursor.arraysize and frame.f_trace_opcodes gain the deletion check.

• Issue: Argument Clinic: Better internal implementation for @getter and @setter #113318