install only compatible versions of Pyomo and pyutilib

setup.py

@@ -60,8 +60,13 @@ def read(*rnames):
         'production cost', 'capacity expansion',
         'planning', 'optimization'
     ],
+    python_requires='>=2.7.12',
     install_requires=[
-        'Pyomo>=4.4.1', # We need a version that works with glpk 4.60+
+        # Pyomo 4.4.1+ works with glpk 4.60+
+        'Pyomo >=4.4.1, <=5.6.8',
+        # pyutilib 6.0 breaks compatibility, and earlier versions of Pyomo
+        # will cheerfully install it, so we explicitly block it
+        'pyutilib <=5.7.3',
         'pint',         # needed by Pyomo when we run our tests, but not included
         'testfixtures', # used for standard tests
         'pandas',       # used for input upgrades and testing that functionality