Boto has a an issue with https and non-default regions. Bug report here: https://github.com/boto/boto/issues/621
A workaround for this bug is to specify the specific s3 host when creating S3Connection, e.g.
S3Connection(access_key, secret_key, calling_format=calling_format, host="s3-eu-west-1.amazonaws.com")
It would be nice to add an AWS_S3_HOST setting and pass the value of this to S3Connection