Add and fix more type hints

Summary

Related to #192 (closed)

Adding more type hints to smaller modules than models.py

Focus

Point the reviewer to the core of the code change. Where should they start reading? What should they focus on (e.g. security, performance, maintainability, user-friendliness, compliance with the specs, finding more corner cases, concrete questions)?

Test Environment

How to set up a test environment for manual testing?

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.

src/linkahead/connection/authentication/external_credentials_provider.py

@@ -23,13 +23,10 @@
 #
 """external_credentials_provider."""
 from __future__ import absolute_import, unicode_literals
-from abc import ABCMeta
+from abc import ABC
 import logging
 from .plain import PlainTextCredentialsProvider
 
-# meta class compatible with Python 2 *and* 3:
-ABC = ABCMeta(str('ABC'), (object, ), {str('__slots__'): ()})
-
 
 class ExternalCredentialsProvider(PlainTextCredentialsProvider, ABC):
     """ExternalCredentialsProvider.