Replace use of apptools sweetpickle

.travis.yml

@@ -10,8 +10,6 @@ env:
 
 matrix:
   include:
-    - env: RUNTIME=2.7
-    - env: RUNTIME=3.5
     - env: RUNTIME=3.6
   fast_finish: true
 

appveyor.yml

@@ -7,8 +7,6 @@ environment:
 
   matrix:
 
-    - RUNTIME: '2.7'
-    - RUNTIME: '3.5'
     - RUNTIME: '3.6'
 
 matrix:

codetools/contexts/data_context.py

@@ -14,8 +14,9 @@
 
 from collections import MutableMapping as DictMixin
 from contextlib import contextmanager
+import pickle
 
-from apptools import sweet_pickle
+from apptools.persistence.versioned_unpickler import VersionedUnpickler
 from traits.adaptation.api import (
     AdaptationOffer, get_global_adaptation_manager)
 from traits.api import (
@@ -191,7 +192,7 @@ def load(file_or_path):
             file_object = open(file_or_path, 'rb')
 
         try:
-            data_context = sweet_pickle.load(file_object)
+            data_context = VersionedUnpickler(file_object).load()
         finally:
             if should_close:
                 file_object.close()
@@ -224,7 +225,7 @@ def save(self, file_or_path):
             for item in self.keys():
                 if isinstance(self[item], tuple(NonPickleable)):
                     del self[item]
-            sweet_pickle.dump(self, file_object, 1)
+            pickle.dump(self, file_object, 1)
         finally:
             if should_close:
                 file_object.close()

etstool.py

@@ -103,7 +103,7 @@
     "mock",
 }
 
-supported_runtimes = ["2.7.13", "3.5.2", "3.6.0"]
+supported_runtimes = ["3.6"]
 
 @click.group()
 def cli():