make generating ...property DRY.

[junkmd]

As mentioned in #367, I changed to reference attributes of _MemberSpec instead of unpacking it.

Adding type annotations to _make_methods or _make_methods argument(methods in line 650) will cause errors in the static type checker.
Because methods variables are overwritten(methods in line 770) in the function.

To solve this, I planned to assign them to different named variables.
However, the same process was used where the type checker error raised.
Therefore I separated the process into PropertyMapping class, made variables not overwritten and made the class called from both methods.