Update pvsystem.py
Update pvsystem.py

I think you need this change in tkinter module import in order to comply with Python 3. 

Cheers,
Elena
pvlib/pvlib-python,ianctse/pvlib-python,wholmgren/pvlib-python,cwhanse/pvlib-python,anomam/pvlib-python,rubennj/pvlib-python,alorenzo175/pvlib-python,uvchik/pvlib-python,mikofski/pvlib-python