Bug 1717645: Fix up_to_date for Python 3.6 r=ahal

Python 3.6 doesn't support the `capture_output` option, so use
`stdout=subprocess.PIPE` instead.

Differential Revision: https://phabricator.services.mozilla.com/D120027

python/mozbuild/mozbuild/virtualenv.py

@@ -205,7 +205,7 @@ class VirtualenvManager(VirtualenvHelper):
 
         if env_requirements.pypi_requirements:
             pip_json = self._run_pip(
-                ["list", "--format", "json"], capture_output=True
+                ["list", "--format", "json"], stdout=subprocess.PIPE
             ).stdout
             installed_packages = json.loads(pip_json)
             installed_packages = {