Bug 1504308 - [mach] Pass path to Python interpreter to python-tests.…

… r=ahal

Differential Revision: https://phabricator.services.mozilla.com/D25760

--HG--
extra : moz-landing-system : lando

python/mach_commands.py

@@ -76,7 +76,6 @@ def python(self, no_virtualenv, exec_file, args):
                      action='store_true',
                      help='Verbose output.')
     @CommandArgument('--python',
-                     default='2.7',
                      help='Version of Python for Pipenv to use. When given a '
                           'Python version, Pipenv will automatically scan your '
                           'system for a Python that matches that given version.')
@@ -109,6 +108,7 @@ def run_python_tests(self,
                          jobs=None,
                          python=None,
                          **kwargs):
+        python = python or self.virtualenv_manager.python_path
         self.activate_pipenv(pipfile=None, populate=True, python=python)
 
         if test_objects is None: