Microrefactoring: py_version <= 2 should be py_version == 2 (python#11986)

Author: sobolevn

mypy/checkexpr.py

@@ -2205,7 +2205,7 @@ def visit_op_expr(self, e: OpExpr) -> Type:
                     return self.strfrm_checker.check_str_interpolation(e.left, e.right)
                 if isinstance(e.left, StrExpr):
                     return self.strfrm_checker.check_str_interpolation(e.left, e.right)
-            elif pyversion[0] <= 2:
+            elif pyversion[0] == 2:
                 if isinstance(e.left, (StrExpr, BytesExpr, UnicodeExpr)):
                     return self.strfrm_checker.check_str_interpolation(e.left, e.right)
         left_type = self.accept(e.left)