execfile in python3 breaks emacs mode

[Rustom Mody]

Removing execfile from python3 has broken the good-ol python-mode of emacs. 

Changing the line

In python-mode.el in function py-execute-file changing the line
(cmd (format "execfile(r'%s') # PYTHON-MODE\n" filename)))

to
 (cmd (format "exec(open(r'%s').read()) # PYTHON-MODE\n" filename)))

seems to solve the problem

Since I am not quite upto the subtleties of what all has changed vis-a-vis exec/execfile, please inform me if this is ok