Calculate settings for S3Boto3StorageFile when instantiated, not impo…

…rted (#930)

This allows the "storages.backends.s3boto3" module to be cleanly imported
before Django settings are configured. This further supports the changes
from #524.

Removing "S3Boto3StorageFile.buffer_size" as a class variable does not
affect the API surface of "S3Boto3StorageFile", as "buffer_size" was
always accessible as an instance variable.

storages/backends/s3boto3.py

@@ -75,7 +75,6 @@ def _cloud_front_signer_from_pem(key_id, pem):
 
 @deconstructible
 class S3Boto3StorageFile(File):
-
     """
     The default file object used by the S3Boto3Storage backend.
 
@@ -92,7 +91,6 @@ class S3Boto3StorageFile(File):
     order to properly write the file to S3. Be sure to close the file
     in your application.
     """
-    buffer_size = setting('AWS_S3_FILE_BUFFER_SIZE', 5242880)
 
     def __init__(self, name, mode, storage, buffer_size=None):
         if 'r' in mode and 'w' in mode:
@@ -113,8 +111,7 @@ def __init__(self, name, mode, storage, buffer_size=None):
         # Amazon allows up to 10,000 parts.  The default supports uploads
         # up to roughly 50 GB.  Increase the part size to accommodate
         # for files larger than this.
-        if buffer_size is not None:
-            self.buffer_size = buffer_size
+        self.buffer_size = buffer_size or setting('AWS_S3_FILE_BUFFER_SIZE', 5242880)
         self._write_counter = 0
 
     @property