Use highest possible pickle protocol

jedi/_compatibility.py

@@ -446,9 +446,6 @@ if sys.version_info[:2] == (3, 3):
     pickle.loads = loads
 
 
-_PICKLE_PROTOCOL = 2
-
-
 def pickle_load(file):
     try:
         if is_py3:
@@ -462,9 +459,9 @@ def pickle_load(file):
         raise
 
 
-def pickle_dump(data, file):
+def pickle_dump(data, file, protocol):
     try:
-        pickle.dump(data, file, protocol=_PICKLE_PROTOCOL)
+        pickle.dump(data, file, protocol)
         # On Python 3.3 flush throws sometimes an error even though the writing
         # operation should be completed.
         file.flush()
@@ -476,6 +473,20 @@ def pickle_dump(data, file):
         raise
 
 
+# Determine the highest protocol version compatible for a given list of Python
+# versions.
+def highest_pickle_protocol(python_versions):
+    protocol = 4
+    for version in python_versions:
+        if version[0] == 2:
+            # The minimum protocol version for the versions of Python that we
+            # support (2.7 and 3.3+) is 2.
+            return 2
+        if version[1] < 4:
+            protocol = 3
+    return protocol
+
+
 try:
     from inspect import Parameter
 except ImportError: