Fixed an error in frame.py
Fixed an error in frame.py

There was a spot in frame.py where keys were being deleted from a
dictionary in a for loop over the keys of that dictionary. This threw up
errors and has now been fixed.

sampadsaha5/sympy,chaffra/sympy,madan96/sympy,kaushik94/sympy,chaffra/sympy,aktech/sympy,kaushik94/sympy,aktech/sympy,souravsingh/sympy,madan96/sympy,skidzo/sympy,jaimahajan1997/sympy,skidzo/sympy,aktech/sympy,yashsharan/sympy,jaimahajan1997/sympy,sampadsaha5/sympy,sampadsaha5/sympy,yashsharan/sympy,postvakje/sympy,yashsharan/sympy,souravsingh/sympy,kaushik94/sympy,jaimahajan1997/sympy,postvakje/sympy,madan96/sympy,skidzo/sympy,souravsingh/sympy,postvakje/sympy,chaffra/sympy