enable assignment operators and add substraction support for numbers

jedi/evaluate/precedence.py

@@ -202,4 +202,7 @@ def _element_calculate(left, operator, right):
     elif operator == '+':
         if is_number(left) and is_number(right) or is_string(left) and is_string(right):
             return [create(left.obj + right.obj)]
+    elif operator == '-':
+        if is_number(left) and is_number(right):
+            return [create(left.obj - right.obj)]
     return [left, right]