MNT: Update test image thresholds for Matplotlib 3.3

Slight change to rotation calculation results in subtle pixel-level
changes, so update the tolerances--the changes aren't big enough to
warrant new images.

tests/plots/test_declarative.py

@@ -408,7 +408,7 @@ def test_declarative_barb_earth_relative():
     return pc.figure
 
 
-@pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.347)
+@pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.607)
 def test_declarative_barb_gfs():
     """Test making a contour plot."""
     data = xr.open_dataset(get_test_data('GFS_test.nc', as_file_obj=False))
@@ -436,7 +436,8 @@ def test_declarative_barb_gfs():
     return pc.figure
 
 
-@pytest.mark.mpl_image_compare(remove_text=True, tolerance=0.022)
+@pytest.mark.mpl_image_compare(remove_text=True,
+                               tolerance={'2.1': 0.407}.get(MPL_VERSION, 0.022))
 def test_declarative_sfc_obs():
     """Test making a surface observation plot."""
     data = pd.read_csv(get_test_data('SFC_obs.csv', as_file_obj=False),