Use warnings.warn, not logging.warning, in microseconds locator warning.

anntzer · anntzer · commit 14a501c5fe4d · 2019-08-07T14:22:09.000+02:00
Otherwise something like
```
from pylab import *
from matplotlib import dates; from datetime import *
plt.plot([datetime(1990, 1, 1), datetime(1990, 1, 1) + timedelta(microseconds=1500)], [0, 0])
plt.show()
```
results in the same warning being emitted multiple times *for every
draw* (e.g. when zooming/panning), which is pretty needless spam.
diff --git a/lib/matplotlib/dates.py b/lib/matplotlib/dates.py
@@ -1457,9 +1457,10 @@ def get_locator(self, dmin, dmax):
         else:
             locator = MicrosecondLocator(interval, tz=self.tz)
             if dmin.year > 20 and interval < 1000:
-                _log.warning('Plotting microsecond time intervals is not well '
-                             'supported. Please see the MicrosecondLocator '
-                             'documentation for details.')
+                cbook._warn_external(
+                    'Plotting microsecond time intervals is not well '
+                    'supported; please see the MicrosecondLocator '
+                    'documentation for details.')
 
         locator.set_axis(self.axis)
 
