Small fix for Google App Engine
Small fix for Google App Engine

On GAE you get this error 
Exception Type: NotImplementedError
Exception Value: Only tempfile.TemporaryFile is available for use

With this fix, xlsxwriter use in_memory mode, so all work good
epantry/django-sql-explorer,groveco/django-sql-explorer,groveco/django-sql-explorer,epantry/django-sql-explorer,groveco/django-sql-explorer,groveco/django-sql-explorer,epantry/django-sql-explorer