Plot failures under matplotlib 3.x

Under 2.x, there are warnings in the report jobs making plots. Example:

2020-06-01 15:55:11,161 | kw_test_py3-report : INFO : generating calibration distribution plots
/home/patrick.godwin/.conda/envs/idq/lib/python3.7/site-packages/matplotlib/axes/_axes.py:6571: UserWarning: The 'normed' kwarg is deprecated, and has been replaced by the 'density' kwarg.
  warnings.warn("The 'normed' kwarg is deprecated, and has been "