Merge pull request #1338 from mraspaud/fix-wavelength-class-defaults-3.6

Fix exception to catch when new namedtuple syntax is used

satpy/dataset/dataid.py

@@ -75,7 +75,7 @@ def __repr__(self):
 
 try:
     wlklass = namedtuple("WavelengthRange", "min central max unit", defaults=('µm',))
-except NameError:  # python 3.6
+except TypeError:  # python 3.6
     wlklass = namedtuple("WavelengthRange", "min central max unit")
     wlklass.__new__.__defaults__ = ('µm',)