Class attribute inheritance deviation

In a hierarchy of classes having class attributes, updating the class attribute in a super class is seen by the subclasses. Below `B.p` resolves to the updated value of `p` on `A`.

```python
class A:
p = 1

class B(A):
pass

print(B.p)
# 1

A.p = 2

print(B.p)
# 2
```

This is not how Param behaves, updating `p` on `A` has no effect on the subclasses. Not sure to which extent this is a bug.

```python
import param

class A(param.Parameterized):
p = param.Parameter(1)

class B(A):
p = param.Parameter()

print(B.p)
# 1

A.p = 2

print(B.p)
# (Different behavior)
# 1
```