Make test_change_epoch more robust.

The epoch is set by explicitly calling `mdates.set_epoch`, or implicitly
set to the default by `mdates.get_epoch` (which is also called when
converting datetimes.)

However, when running tests in parallel, there is no guarantee that any
other call to `[gs]et_epoch` might have been made to lock it in, so we
need to do that explicitly.

Author: QuLogic

lib/matplotlib/tests/test_dates.py

@@ -1095,12 +1095,15 @@ def test_datetime64_in_list():
 def test_change_epoch():
     date = np.datetime64('2000-01-01')
 
+    # use private method to clear the epoch and allow it to be set...
+    mdates._reset_epoch_test_example()
+    mdates.get_epoch()  # Set default.
+
     with pytest.raises(RuntimeError):
         # this should fail here because there is a sentinel on the epoch
         # if the epoch has been used then it cannot be set.
         mdates.set_epoch('0000-01-01')
 
-    # use private method to clear the epoch and allow it to be set...
     mdates._reset_epoch_test_example()
     mdates.set_epoch('1970-01-01')
     dt = (date - np.datetime64('1970-01-01')).astype('datetime64[D]')