Propagate global interpreter constraints when building PEXes with interpreter constraints requested

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

@@ -255,8 +255,9 @@ 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
-    for tgt in constraint_tgts:
-      for constraint in tgt.compatibility:
+    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):

src/python/pants/backend/python/tasks/python_binary_create.py

@@ -140,7 +140,8 @@ def _create_binary(self, binary_tgt, results_dir):
         if is_python_target(tgt):
           constraint_tgts.append(tgt)
 
-      # Add target's interpreter compatibility constraints to pex info.
+      # Add interpreter compatibility constraints to pex info. This will first check the targets for any
+      # constraints, and if they do not have any will resort to the global constraints.
       pex_builder.add_interpreter_constraints_from(constraint_tgts)
 
       # Dump everything into the builder's chroot.