Implement @sym/class to return the Python class

[cbm755]

In PyTave and Java class(x) gives the Python or Java class. That might be nice for us too.

E.g.,

a = interval(1, 5);
class(a)
  ans = sympy.Interval 

For now, it would be a string. Later, a pyobject...

No pressing reason to do this now, but maybe its fun to think about.