Generify solution by using compatibility_or_constraints()

Instead of applying a bandaid for only `./pants binary`, John proposed fixing the issue with our PexBuilderWrapper itself. So, we use `compatibility_or_constrains()`, which will first try to return the target's compatibility, else will return the Python Setup subystem's value.

The wrapper still is not ideal and John proposes killing add_interpreter_constraint() and add_interpreter_constraints_from() to instead automatically be setting the interpreter constraints from the targets graph. This PR does not make that change for the scope, but this should be noted.

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

@@ -255,9 +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:
-        self.add_interpreter_constraint(constraint)
+    constraints = {self._python_setup_subsystem.compatibility_or_constraints(tgt) for tgt in constraint_tgts}
+    for constraint in constraints:
+      self.add_interpreter_constraint(constraint)
 
   def add_direct_requirements(self, reqs):
     for req in reqs:

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

@@ -143,7 +143,6 @@ def _create_binary(self, binary_tgt, results_dir):
 
       # Add global and target-level interpreter compatibility constraints to pex info.
       pex_builder.add_interpreter_constraints_from(constraint_tgts)
-      pex_builder.add_interpreter_constraint(PythonSetup.global_instance().interpreter_constraints)
 
       # Dump everything into the builder's chroot.
       for tgt in source_tgts: