Bug 1730712: Python tests should use Python 3 automatically r=ahal

The virtualenv python is always Python 3, so inline the `version_info()`
function.

Differential Revision: https://phabricator.services.mozilla.com/D129324

python/mach_commands.py

@@ -217,12 +217,7 @@ def run_python_tests(
     elif subsuite:
         filters.append(mpf.subsuite(subsuite))
 
-    tests = mp.active_tests(
-        filters=filters,
-        disabled=False,
-        python=command_context.virtualenv_manager.version_info()[0],
-        **mozinfo.info
-    )
+    tests = mp.active_tests(filters=filters, disabled=False, python=3, **mozinfo.info)
 
     if not tests:
         submsg = "for subsuite '{}' ".format(subsuite) if subsuite else ""