Update noxfile.py (#207)

noxfile.py

@@ -21,7 +21,7 @@ def docs_test(session):
     """
     Same as `docs`, but rebuild everything and fail on warnings for testing
     """
-    session.install("-r", "requirements.txt")
+    session.install("-e", ".")
     cmd = ["sphinx-build"]
     cmd.extend(['-W', '--keep-going', '-E', '-a'])
     cmd.extend(build_command + session.posargs)