Fix imports for DRF 3.10 (#408)

Fixes #410
Fixes #411

src/drf_yasg/generators.py

@@ -4,14 +4,23 @@ import re
 from collections import OrderedDict, defaultdict
 
 import uritemplate
+import rest_framework
 from coreapi.compat import urlparse
 from rest_framework import versioning
 from rest_framework.compat import URLPattern, URLResolver, get_original_route
 from rest_framework.schemas.generators import EndpointEnumerator as _EndpointEnumerator
-from rest_framework.schemas.generators import SchemaGenerator, endpoint_ordering, get_pk_name
-from rest_framework.schemas.inspectors import get_pk_description
+from rest_framework.schemas.generators import endpoint_ordering, get_pk_name
+
 from rest_framework.settings import api_settings
 
+from packaging.version import Version
+if Version(rest_framework.__version__) < Version('3.10'):
+    from rest_framework.schemas.generators import SchemaGenerator
+    from rest_framework.schemas.inspectors import get_pk_description
+else:
+    from rest_framework.schemas import SchemaGenerator
+    from rest_framework.schemas.utils import get_pk_description
+
 from . import openapi
 from .app_settings import swagger_settings
 from .errors import SwaggerGenerationError