Add test for pex exec with just constraints

tests/test_integration.py

@@ -537,6 +537,24 @@ def test_plain_pex_exec_no_ppp_no_pp_no_constraints():
     assert str(sys.executable).encode() in stdout
 
 
+@pytest.mark.skipif(NOT_CPYTHON_36)
+def test_plain_pex_exec_with_just_constraints():
+  with temporary_dir() as td:
+    pex_out_path = os.path.join(td, 'pex.pex')
+    res = run_pex_command(['--disable-cache',
+      '--interpreter-constraint=>3',
+      '--interpreter-constraint=<3.8',
+      '-o', pex_out_path])
+    res.assert_success()
+
+    stdin_payload = b'import sys; print(sys.executable); sys.exit(0)'
+    stdout, rc = run_simple_pex(pex_out_path, stdin=stdin_payload)
+    assert rc == 0
+    str_version = str(stdout).split(" ")[1].split('.')  # parse interpreter version
+    version = tuple(map(lambda x: int(x), str_version))
+    assert (3, 0, 0) < version < (3, 8, 0)
+
+
 def test_pex_exec_with_pex_python_path_only():
   ensure_python_interpreter('2.7.10')
   ensure_python_interpreter('3.6.3')