pprint does not handle circular references

[philippjfr]

When there are circular references on a Parameterized object the pretty printing causes a recursion error:

class A(param.Parameterized):
    
    b = param.Parameter()
    
class B(param.Parameterized):
    
    a = param.Parameter()
    
a = A()
b = B(a=a)
a.b = b
a.pprint()

I'd suggest there should be a maximum depth for the pretty printer, anything more than 1 or two levels deep will become unreadable anyway.

[jbednar]

Fixed in #499.