sympy.sets: Add is_proper_subset() and is_proper_superset() methods

Having these methods will not force one to make use of the < or >
operators for these operations only, since we already have is_subset()
and is_superset()  methods.
Merge pull request #7520 from amitsaha/proper_subset_superset

sympy.sets: Add is_proper_subset() and is_proper_superset() methods