Matplotlib plot_date changed the zero point of the dates in >v3.3

Description

astropy.time.TimePlotDate returns Matplotlib plot_date input: 1 + number of days from 0001-01-01 00:00:00 UTC But Matplotlib changed the date from which they count in plot_date function to 1970-01-01 UTC since Matplotlib v3.3 See the note here: https://matplotlib.org/api/dates_api.html#matplotlib-date-format

Expected behavior

import matplotlib.pyplot as plt 
from astropy.time import Time 
import numpy as np 
jyear = np.linspace(2000, 2001, 20) 
t = Time(jyear, format='jyear', scale='utc') 
plt.plot_date(t.plot_date, jyear)                  
plt.show()

Should plot dates 2000 to 2001

Actual behavior

Instead, resulted in a plot showing dates from the year 3969 to 3970

Steps to Reproduce

1. Install lastest version of astropy and matplotlib
2. Try the code snippet below

import matplotlib.pyplot as plt 
from astropy.time import Time 
import numpy as np 
jyear = np.linspace(2000, 2001, 20) 
t = Time(jyear, format='jyear', scale='utc') 
plt.plot_date(t.plot_date, jyear)                  
plt.show()

System Details

Linux-3.10.0-957.1.3.el7.x86_64-x86_64-with-centos-7.6.1810-Core Python 3.6.6 (default, Aug 13 2018, 18:24:23) [GCC 4.8.5 20150623 (Red Hat 4.8.5-28)] Numpy 1.17.3 astropy 4.1 Scipy 1.5.2 Matplotlib 3.3.3

[indiajoe]

Oh…okay… no problem. I shall update to python 3.7 or later

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