Fix creating empty id field in models

[i8enn]

• Fix #23 issue
• Created MongoDB decoders for decode MongoDB documents to internal pydantic mixin models.
• Replaced faked id property to normally pydantic field
• Implemented decoding mongo documents and data in _doc field with MongoDB decoders
• Replaced _id field to id in _doc mixin field
• Removed unused methods, functions and properties from mixins and mixins tests
• Moved pytest and coverage configurations to setup.cfg
• Changed pytest config:
  • Changed test classes pattern to *TestCase
  • Changed test functions pattern to test_*
  • Changed finding tests to finding in all python files in tests directory
  • Changed watching directories for looponfile mode to tests and pydantic-odm