Added as_super and into_super methods for Bound<T: PyClass>

[JRRudy1]

This PR adds two new methods to provide a more convenient mechanism for casting a Bound pyclass to its base type. The new methods mirror the corresponding methods for PyRef and PyRefMut (see #4219).

Casting Bound<T> to Bound<T::BaseType> can be useful in the context of the PyMyClassMethods idiom when an extension trait is use to implemented methods (for use from Rust) on the Bound version of a pyclass's base type. The upcast of a pyclass to its base type is a trivially safe operation, but making this cast is currently extremely verbose.

For example, there are currently two options for casting Bound<T> to Bound<T::BaseType by value:

// 3 method calls including an unnecessary `unwrap`
let sub: Bound<SubClass> = /* ... */;
let _ = sub.into_any().downcast_into::<BaseClass>().unwrap(); 
// or 2 method calls using `unsafe`
let sub: Bound<SubClass> = /* ... */;
let _ = unsafe { sub.into_any().downcast_into_unchecked::<BaseClass>() };

Upcasting by reference is a bit less painful since you can leverage Deref<Target=PyAny> to avoid the first method call, but its still ugly and requires either an unwrap or an unsafe block.

The new methods added by this PR make it much nicer:

let sub: Bound<SubClass> = /* ... */;
// upcast by reference
let _= sub.as_super(); 
// upcast by value
let _ = sub.into_super();

[JRRudy1]

Good catches, thanks!