Fix type error in titer model cross-validation

Fixes type errors in titer model code that is only accessible through
the developer's Python API but which prevented me from running
cross-validation of titer models with that API. It looks like these were
the only references to the dictionary methods of keys and values that
didn't get updated in the past.

augur/titer_model.py

@@ -226,7 +226,7 @@ def __init__(self, titers, **kwargs):
         else:
             self.titers = titers
             strain_counts = type(self).count_strains(titers)
-            self.strains = strain_counts.keys()
+            self.strains = list(strain_counts.keys())
 
     def read_titers(self, fname):
         self.titer_fname = fname
@@ -504,7 +504,7 @@ def validate(self, plot=False, cutoff=0.0, validation_set = None, fname=None):
             pred_titer = self.predict_titer(key[0], key[1], cutoff=cutoff)
             validation[key] = (val, pred_titer)
 
-        validation_array = np.array(validation.values())
+        validation_array = np.array(list(validation.values()))
         actual = validation_array[:,0]
         predicted = validation_array[:,1]
 
@@ -517,7 +517,7 @@ def validate(self, plot=False, cutoff=0.0, validation_set = None, fname=None):
                         'rms_error': np.sqrt(np.mean((actual-predicted)**2)),
         }
         pprint(model_performance)
-        model_performance['values'] = validation.values()
+        model_performance['values'] = list(validation.values())
 
         self.validation = model_performance