Fix tests for Python 3.6

Makefile

@@ -27,6 +27,7 @@ docs-dev:
 
 install-deps:
 	$(PIP) install -i https://pypi.gurobi.com gurobipy
+	$(PIP) install xpress
 	$(PIP) install -r requirements.txt
 
 install:

miplearn/types.py

@@ -2,7 +2,9 @@
 #  Copyright (C) 2020, UChicago Argonne, LLC. All rights reserved.
 #  Released under the modified BSD license. See COPYING.md for more details.
 
-from typing import TypedDict, Optional, Dict, Callable, Any
+from typing import Optional, Dict, Callable, Any
+
+from mypy_extensions import TypedDict
 
 TrainingSample = TypedDict(
     "TrainingSample",