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
```
issue