check if underlying file object is closed in closed method

[jordancrane]

Currently it appears that S3Boto3StorageFile uses the closed property defined by FileProxyMixin, which checks the condition not self.file or self.file.closed (source). Since self.file in S3Boto3StorageFile is a property that delegates to _get_file, this check reopens the file. Thus closed always returns False.

[jschneier]

Thanks, can you fix the linting issue.

Also, let's add the beginnings of a test class for S3Boto3StorageFile that covers this fix.

[jschneier]

Thanks!