Deprecate automatically registering matplotlib units (partial revert of 0.21.0)
Given the feedback we have got (https://github.com/pandas-dev/pandas/issues/18153, https://github.com/pandas-dev/pandas/issues/18192, https://github.com/pandas-dev/pandas/issues/18212,  https://github.com/pandas-dev/pandas/issues/18283, https://github.com/matplotlib/matplotlib/issues/9577, https://github.com/matplotlib/matplotlib/issues/9610, https://github.com/matplotlib/matplotlib/issues/9771, https://github.com/pydata/xarray/issues/166), it seems we underestimated the impact and we should consider (partly) reverting this change to properly deprecate it instead. 

This should also give matplotlib the time to implement basic datetime64 support (https://github.com/matplotlib/matplotlib/issues/9610, https://github.com/matplotlib/matplotlib/pull/9779)

Depending on how we can do this, this might have the consequence that have to undo temporarily the lazy import of matplotlib (affecting the import time). See also discussion in https://github.com/pandas-dev/pandas/issues/18283

Opening this issue to keep track of it, should be decided/done for 0.21.1