text: Title: `ImportError: Module "drf_yasg.generators" does not define a "OpenAPISchemaGenerator" attribute/class` after upgrading DRF==3.14.0
  Body: # Bug Report
  
  ## Description
  
  After update Django Rest Framework to 3.14.0, Django did not start, because drf-yasg raise exception `ImportError: Could not import 'drf_yasg.generators.OpenAPISchemaGenerator' for API setting 'DEFAULT_GENERATOR_CLASS'. ` Reverting to drf==3.13.1 resolves the issue.
  
  ## Is this a regression?
  
  Not sure, there was a similar issue here: https://github.com/axnsan12/drf-yasg/issues/641
  
  ## Minimal Reproduction
  
  ```python
  # settings.py
  
  INSTALLED_APPS = [
     ...  
    "drf_yasg",
  ]
  
  SWAGGER_SETTINGS = {
      "SECURITY_DEFINITIONS": {
          "Bearer": {
              "type": "apiKey",
              "in": "header",
              "name": "Authorization",
              "template": "Bearer {apiKey}",
          },
      },
      "DEFAULT_FIELD_INSPECTORS": [
          "drf_yasg.inspectors.CamelCaseJSONFilter",
          "drf_yasg.inspectors.InlineSerializerInspector",
          "drf_yasg.inspectors.RelatedFieldInspector",
          "drf_yasg.inspectors.ChoiceFieldInspector",
          "drf_yasg.inspectors.FileFieldInspector",
          "drf_yasg.inspectors.DictFieldInspector",
          "drf_yasg.inspectors.SimpleFieldInspector",
          "drf_yasg.inspectors.StringDefaultFieldInspector",
      ],
  }
  ```
  
  ```python
  # urls.py
  from drf_yasg import openapi
  from drf_yasg.views import get_schema_view
  from rest_framework import permissions
  
  schema_view = get_schema_view(
      openapi.Info(
          title="My API",
          default_version="v1",
          description="My API",
          terms_of_service="https://www.google.com/policies/terms/",
          contact=openapi.Contact(email="leohakim@gmail.com"),
          license=openapi.License(name="BSD License"),
      ),
      public=True,
      permission_classes=(permissions.AllowAny,),
  )
  ```
  
  ## Stack trace / Error message
  
  ```code
    File "/usr/local/lib/python3.9/site-packages/drf_yasg/views.py", line 67, in get_schema_view
      _generator_class = generator_class or swagger_settings.DEFAULT_GENERATOR_CLASS
    File "/usr/local/lib/python3.9/site-packages/drf_yasg/app_settings.py", line 122, in __getattr__
      val = perform_import(val, attr)
    File "/usr/local/lib/python3.9/site-packages/rest_framework/settings.py", line 166, in perform_import
      return import_from_string(val, setting_name)
    File "/usr/local/lib/python3.9/site-packages/rest_framework/settings.py", line 180, in import_from_string
      raise ImportError(msg)
  ImportError: Could not import 'drf_yasg.generators.OpenAPISchemaGenerator' for API setting 'DEFAULT_GENERATOR_CLASS'. ImportError: Module "drf_yasg.generators" does not define a "OpenAPISchemaGenerator" attribute/class.
  ```
  
  ## Your Environment
  
  ```code
  drf-yasg=1.21.3
  djangorestframework=3.14.0
  django=4.1.1
  ```
  