Consider add typing system for parselmouth? Maybe some stub files?

[fncokg]

It's sometimes useful to enable the typing system for static type checking. However, currently, parselmouth does not support typing system so that we can only see parselmouth objects as Any types, which is harmful for all the other codes that have been annotated types.

To enable the typing system of parselmouth via stub files is extremely easy: all we need is to do some modifications on the stubgen-generated stub files and this is also the choice of numpy, scipy, pandas etc.