Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>

The defusedxml package contains several Python-only workarounds and
fixes for denial of service and other vulnerabilities in Python's XML
libraries. In order to benefit from the protection you just have to
import and use the listed functions / classes from the right defusedxml
module instead of the original module.

Currently, openpyxl detects, and uses defusedxml, if installed.

Signed-off-by: Eneas U de Queiroz <cote2004-github@yahoo.com>