Remove deprecation warning from defusedxml
*Created originally on Bitbucket by [Álvaro Justen - Turicas (Álvaro Justen)](https://bitbucket.org/%7B25d4071b-3b79-4bd3-ad0c-8b45ad8be33d%7D/)* `defusedxml.lxml` will be removed in the future and it's currently generating this warning: ``` /home/turicas/software/pyenv/versions/3.7.3/envs/rows/lib/python3.7/site-packages/openpyxl/xml/functions.py:31 /home/turicas/software/pyenv/versions/3.7.3/envs/rows/lib/python3.7/site-packages/openpyxl/xml/functions.py:31: DeprecationWarning: defusedxml.lxml is no longer supported and will be removed in a future release. from defusedxml.lxml import fromstring as _fromstring, tostring ```
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