refactor(python): deprecate iterrows in favour of iter_rows, add new @redirect class decorator

[alexander-beedie]

Renames iterrows to iter_rows, consistent with the new iter_slices method.

• Introduces new class decorator @redirect to help generically support a transition period for one or more renamed methods or attributes; triggers a DeprecationWarning on use, while transparently redirecting the underlying object's __getattribute__ to the new method.

• Note that the redirecting code is only called if the requested method/attribute isn't found on the class (due to the differences in behaviour between __getattr__ and __getattribute__) so it adds zero overhead when invoking a method/attribute that exists.

---

New decorator usage:

@redirect({"iterrows": "iter_rows"})
class DataFrame:
    ...

Feels pretty clean; we can support both the new name ...

list( pl.DataFrame([1,2,3]).iter_rows() )
# [(1,), (2,), (3,)]

... and the old name (with an automatically-raised DeprecationWarning) ...

list( pl.DataFrame([1,2,3]).iterrows() )
# [(1,), (2,), (3,)]
#
# DeprecationWarning: `DataFrame.iterrows` has been renamed; this 
#   redirect is temporary, please use `.iter_rows` instead

... while having only the newer method visible on the class:

for attr in dir( pl.DataFrame ):
    if attr.startswith( "iter" ):
        print( attr )
        
# iter_rows
# iter_slices