Install local dependencies when running lint (#4936)

Author: Jon Wayne Parrott

packages/google-cloud-datastore/nox.py

@@ -124,7 +124,7 @@ def lint(session):
     serious code quality issues.
     """
     session.interpreter = 'python3.6'
-    session.install('flake8')
+    session.install('flake8', *LOCAL_DEPS)
     session.install('.')
     session.run('flake8', 'google', 'tests')