Bug 1657719: Don't create init_py3_py3 venv in python-test r=firefox-build-system-reviewers,rstewart

To ensure that a python 3 virtualenv exists for tests, python-test will ensure that it's created (regardless of which version of python is currently in use).
However, the existing logic was incorrectly creating an "extra py3 virtualenv", even if currently running python 3 and having already created a "first" py3 virtualenv

Differential Revision: https://phabricator.services.mozilla.com/D86269

python/mach_commands.py

@@ -256,19 +256,19 @@ class MachCommands(MachCommandBase):
         self.activate_pipenv(os.path.dirname(default_manager.virtualenv_root),
                              pipfile=None, populate=True, python=python)
 
-        # The current process might be running under Python 2 and the Python 3
-        # virtualenv will not be set up by mach bootstrap. To avoid problems in tests
+        # If the current process is running under Python 2, the Python 3
+        # virtualenv may not be set up yet. To avoid problems in tests
         # that implicitly depend on the Python 3 virtualenv we ensure the Python 3
         # virtualenv is up to date before the tests start.
-        python3, version = find_python3_executable(min_version='3.5.0')
-
-        py3_manager = VirtualenvManager(
-            default_manager.topsrcdir,
-            py3_root,
-            default_manager.log_handle,
-            default_manager.manifest_path,
-        )
-        py3_manager.ensure(python3)
+        if not six.PY3:
+            python3, version = find_python3_executable(min_version='3.5.0')
+            py3_manager = VirtualenvManager(
+                default_manager.topsrcdir,
+                py3_root,
+                default_manager.log_handle,
+                default_manager.manifest_path,
+            )
+            py3_manager.ensure(python3)
 
     def _run_python_test(self, test):
         from mozprocess import ProcessHandler