Remove CachedStaticFilesStorage and dependent storages in Django >= 3.1

Importing the referenced class was resulting in an ImportError.
CachedStaticFilesStorage has been deprecated since Django 2.2 and was
finally removed in Django 3.1.

PipelineCachedStorage and NonPackagingPipelineCachedStorage are also
made undefined for in this commit.

Django >= 3.1 users should instead use PipelineManifestStorage and
NonPackagingPipelineManifestStorage which make use of
ManifestStaticFilesStorage.

Fixes issue #714

Author: vmsp

pipeline/storage.py

@@ -2,7 +2,13 @@
 
 from io import BytesIO
 
-from django.contrib.staticfiles.storage import CachedStaticFilesStorage, ManifestStaticFilesStorage, StaticFilesStorage
+import django
+
+_CACHED_STATIC_FILES_STORAGE_AVAILABLE = django.VERSION[0] <= 3 and django.VERSION[1] < 1
+
+if _CACHED_STATIC_FILES_STORAGE_AVAILABLE:
+    from django.contrib.staticfiles.storage import CachedStaticFilesStorage
+from django.contrib.staticfiles.storage import ManifestStaticFilesStorage, StaticFilesStorage
 from django.contrib.staticfiles.utils import matches_patterns
 
 from django.core.files.base import File
@@ -90,14 +96,17 @@ class NonPackagingPipelineStorage(NonPackagingMixin, PipelineStorage):
     pass
 
 
-class PipelineCachedStorage(PipelineMixin, CachedStaticFilesStorage):
-    # Deprecated since Django 2.2
-    pass
+if _CACHED_STATIC_FILES_STORAGE_AVAILABLE:
+    class PipelineCachedStorage(PipelineMixin, CachedStaticFilesStorage):
+        # Deprecated since Django 2.2
+        # Removed in Django 3.1
+        pass
 
 
-class NonPackagingPipelineCachedStorage(NonPackagingMixin, PipelineCachedStorage):
-    # Deprecated since Django 2.2
-    pass
+    class NonPackagingPipelineCachedStorage(NonPackagingMixin, PipelineCachedStorage):
+        # Deprecated since Django 2.2
+        # Removed in Django 3.1
+        pass
 
 
 class PipelineManifestStorage(PipelineMixin, ManifestStaticFilesStorage):