Remove deprecation warning from defusedxml

Created originally on Bitbucket by Álvaro Justen - Turicas (Álvaro Justen)

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

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information