Define the location attribute on base class ProcessEnv (#377)

Fixes errors when running mypy on a project's noxfile.py:

    noxfile.py:42: error: "ProcessEnv" has no attribute "location"

Discovered while adding types pip's noxfile.py:
pypa/pip#9411

nox/virtualenv.py

@@ -43,6 +43,8 @@ def __init__(self, interpreter: str) -> None:
 class ProcessEnv:
     """A environment with a 'bin' directory and a set of 'env' vars."""
 
+    location: str
+
     # Does this environment provide any process isolation?
     is_sandboxed = False