Fix interop pubsub tests and PR feedback

- Use `from_id`, the changed field name in `PSMessage`.
- PR feedbacks
        - Add label `test` in `testenv` in tox.ini, to avoid wrong
dispatching an environment's command in the future.
        - Use `pytest` over `py.test`.

tox.ini

@@ -30,7 +30,7 @@ deps =
 passenv = CI TRAVIS TRAVIS_*
 extras = test
 commands =
-    py.test tests/
+    test: pytest tests/
 basepython =
     py37: python3.7
 
@@ -44,10 +44,12 @@ commands =
     flake8 libp2p tests tests_interop examples setup.py
 
 [testenv:py37-interop]
-deps = p2pclient
+deps =
+    p2pclient
+    pexpect
 passenv = CI TRAVIS TRAVIS_* GOPATH
 extras = test
 commands =
-    py.test tests_interop/
+    pytest tests_interop/
 basepython =
     py37: python3.7