Fix issue with compatibility_or_constraints() returning a tuple

add_interpreter_constraints() expects a str, so we must unpack the tuple when calling it.

src/python/pants/backend/python/subsystems/pex_build_util.py

@@ -255,9 +255,10 @@ def add_interpreter_constraints_from(self, constraint_tgts):
     # TODO this would be a great place to validate the constraints and present a good error message
     # if they are incompatible because all the sources of the constraints are available.
     # See: https://github.com/pantsbuild/pex/blob/584b6e367939d24bc28aa9fa36eb911c8297dac8/pex/interpreter_constraints.py
-    constraints = {self._python_setup_subsystem.compatibility_or_constraints(tgt) for tgt in constraint_tgts}
-    for constraint in constraints:
-      self.add_interpreter_constraint(constraint)
+    constraint_tuples = {self._python_setup_subsystem.compatibility_or_constraints(tgt) for tgt in constraint_tgts}
+    for constraint_tuple in constraint_tuples:
+      for constraint in constraint_tuple:
+        self.add_interpreter_constraint(constraint)
 
   def add_direct_requirements(self, reqs):
     for req in reqs: