Add gevent / boto3 threads warning

Closes etianen#162

django_s3_storage/storage.py

@@ -4,6 +4,8 @@
 import os
 import posixpath
 import shutil
+import sys
+import warnings
 from contextlib import closing
 from datetime import timezone
 from functools import partial, wraps
@@ -198,6 +200,11 @@ def _setup(self):
             raise ImproperlyConfigured(
                 f"Setting AWS_S3_BUCKET_NAME{self.s3_settings_suffix} is required."
             )
+        if "gevent" in sys.modules and self.settings.AWS_S3_USE_THREADS:
+            warnings.warn(
+                "boto3 and gevent interaction likely. "
+                "Please set AWS_S3_USE_THREADS to False."
+            )
         # Create a thread-local connection manager.
         self._connections = _Local(self)
         # Set transfer config for S3 operations