Remove misleading None on error type hint

Problem: The function wait_for_confirmation() has an unreachable "return None". Pylance cannot prove that the return is unreachable and provides an erroneous type hint error

Solution: 
Use PEP 484 annotation to ignore the returned type and fix previous type hints.  

Signed off:
Serhiy: Serhiy1@live.co.uk

Author: serhiy

archivist/assets.py

@@ -109,7 +109,7 @@ def __init__(self, archivist: "type_helper.Archivist"):
 
     def create(
         self, behaviours: List, attrs: Dict, *, confirm: bool = False
-    ) -> NoneOnError[Asset]:
+    ) -> Asset:
         """Create asset
 
         Creates asset with defined behaviours and attributes.
@@ -134,7 +134,7 @@ def create(
 
     def create_from_data(
         self, data: Dict, *, confirm: bool = False
-    ) -> NoneOnError[Asset]:
+    ) -> Asset:
         """Create asset
 
         Creates asset with request body from data stream.
@@ -157,7 +157,7 @@ def create_from_data(
         if not confirm:
             return asset
 
-        return wait_for_confirmation(self, asset["identity"])
+        return wait_for_confirmation(self, asset["identity"])   # type: ignore  The None return is unreachable
 
     def read(self, identity: str) -> Asset:
         """Read asset

archivist/events.py

@@ -25,7 +25,6 @@
 import logging
 from typing import Dict, Optional
 from copy import deepcopy
-from archivist.type_aliases import NoneOnError
 
 # pylint:disable=unused-import      # To prevent cyclical import errors forward referencing is used
 # pylint:disable=cyclic-import      # but pylint doesn't understand this feature
@@ -125,7 +124,7 @@ def create(
         *,
         asset_attrs: Optional[Dict] = None,
         confirm: bool = False,
-    ) -> NoneOnError[Event]:
+    ) -> Event:
         """Create event
 
         Creates event for given asset.
@@ -151,7 +150,7 @@ def create(
 
     def create_from_data(
         self, asset_id: str, data: Dict, *, confirm=False
-    ) -> NoneOnError[Event]:
+    ) -> Event:
         """Create event
 
         Creates event for given asset from data.
@@ -175,7 +174,7 @@ def create_from_data(
         if not confirm:
             return event
 
-        return wait_for_confirmation(self, event["identity"])
+        return wait_for_confirmation(self, event["identity"])   # type: ignore  The None return is unreachable
 
     def read(self, identity: str) -> Event:
         """Read event