Fix a mypy warning

Author: jairov4

mypyc/irbuild/classdef.py

@@ -806,7 +806,9 @@ def create_ne_from_eq(builder: IRBuilder, cdef: ClassDef) -> None:
 
 def gen_glue_ne_method(builder: IRBuilder, cls: ClassIR, line: int) -> None:
     """Generate a "__ne__" method from a "__eq__" method."""
-    eq_sig = cls.get_method("__eq__").decl.sig
+    func_ir = cls.get_method("__eq__")
+    assert func_ir
+    eq_sig = func_ir.decl.sig
     strict_typing = builder.options.strict_dunders_typing
     with builder.enter_method(cls, "__ne__", eq_sig.ret_type):
         rhs_type = eq_sig.args[0].type if strict_typing else object_rprimitive