Replace NotImplemented with NotImplementedError

```
************* Module saml2.authn
src/saml2/authn.py:32:8: E0711: NotImplemented raised - should raise NotImplementedError (notimplemented-raised)
src/saml2/authn.py:32:8: E0702: Raising NotImplementedType while only classes or instances are allowed (raising-bad-type)
src/saml2/authn.py:35:8: E0711: NotImplemented raised - should raise NotImplementedError (notimplemented-raised)
src/saml2/authn.py:35:8: E0702: Raising NotImplementedType while only classes or instances are allowed (raising-bad-type)
src/saml2/authn.py:38:8: E0711: NotImplemented raised - should raise NotImplementedError (notimplemented-raised)
src/saml2/authn.py:38:8: E0702: Raising NotImplementedType while only classes or instances are allowed (raising-bad-type)
```

Signed-off-by: Ivan Kanakarakis <[email protected]>

Author: c00kiemon5ter

src/saml2/authn.py

@@ -29,13 +29,13 @@ def __init__(self, srv):
         self.srv = srv
 
     def __call__(self, *args, **kwargs):
-        raise NotImplemented
+        raise NotImplementedError
 
     def authenticated_as(self, **kwargs):
-        raise NotImplemented
+        raise NotImplementedError
 
     def verify(self, **kwargs):
-        raise NotImplemented
+        raise NotImplementedError
 
 
 def is_equal(a, b):