Refactor OpenAI parser fallback and clean up test imports

Author: jvanderaa

changes/372.fixed

@@ -0,0 +1 @@
+Refactored OpenAI parser to be used as a fallback and fixed subject injection to skip text/calendar parts.

tests/unit/test_providers.py

@@ -7,9 +7,8 @@
 
 from circuit_maintenance_parser.data import NotificationData
 from circuit_maintenance_parser.errors import ProcessorError, ProviderError
-from circuit_maintenance_parser.parser import EmailDateParser, Parser
-from circuit_maintenance_parser.parsers.openai import OpenAIParser
-from circuit_maintenance_parser.processor import CombinedProcessor, SimpleProcessor
+from circuit_maintenance_parser.parser import Parser
+from circuit_maintenance_parser.processor import SimpleProcessor
 from circuit_maintenance_parser.provider import AquaComms, GenericProvider
 
 # pylint: disable=use-implicit-booleaness-not-comparison