Remove six (closes #160)

Climix depends on python3 and does not need python2 compatibility.

climix/util/change_pr_units.py

@@ -4,7 +4,6 @@ from collections import namedtuple, OrderedDict
 
 from cf_units import Unit
 import iris
-import six
 
 
 DENSITY_WATER = Unit('1000 kg m-3')
@@ -40,7 +39,7 @@ def get_standard_name_for_units(units):
 
 
 def ensure_units_and_standard_name(units, standard_name):
-    if isinstance(units, six.string_types):
+    if isinstance(units, str):
         units = Unit(units)
     if units is None and standard_name is None:
         raise ValueError('No new units or new standard name is given.')