Bug 1712819: Fix VirtualenvManager not being expandable in debugger r=ahal

At least in PyCharm, expanding a `VirtualenvManager` instance means
resolving all the properties and fields of the instance.
However, if that property is doing non-trivial work, the debugger
wouldn't run that subprocess while we're stopped at a breakpoint.
So, the instance would sit there with the "Collecting data..." text.

Differential Revision: https://phabricator.services.mozilla.com/D115935

python/mach_commands.py

@@ -222,7 +222,7 @@ class MachCommands(MachCommandBase):
         tests = mp.active_tests(
             filters=filters,
             disabled=False,
-            python=self.virtualenv_manager.version_info[0],
+            python=self.virtualenv_manager.version_info()[0],
             **mozinfo.info
         )