Enable plotting of timedeltas #8869

This is a general issue to track progress in enabling the plotting of timedelta objects. The eventual aim is for code like this to work:

import matplotlib.pyplot as plt
from datetime import timedelta

x = [1, 2]
y = [timedelta(seconds=1), timedelta(seconds=2)]
fig, ax = plt.subplots()
ax.scatter(x, y)
plt.show()

TODO

• Method to convert timedelta to Matplotlib's internal time/date representation (float number of days) - Allow timedelta to be converted to an ordinalf #9120
• Method to convert Matplotlib's internal time/date representation back to timedelta - Add num2timedelta method with test #8870
• Implement a TimedeltaFormatter - timedelta formatter #8930
• Implement a TimedeltaLocator (possibly along with timedelta versions of SecondLocator, MinuteLocator etc.)
• Implement an AutoTimedeltaLocator
• Implement a TimedeltaConverter to go in the units registry