Add tox

- Put extras_require to setup.py
- Add mypy

tox.ini

@@ -0,0 +1,21 @@
+# Reference: https://github.com/ethereum/py_ecc/blob/d0da74402210ea1503ef83b3c489d5b5eba7f7bf/tox.ini
+
+[tox]
+envlist =
+    py37-test
+    lint
+
+[testenv]
+deps =
+extras = test
+commands =
+    pytest --cov=./libp2p tests/
+basepython =
+    py37: python3.7
+
+[testenv:lint]
+basepython = python3
+extras = dev
+commands =
+    pylint --rcfile={toxinidir}/.pylintrc libp2p tests
+    mypy -p p2pclient --config-file {toxinidir}/mypy.ini