Initial python3 changes

Mostly just updating libraries, removing string encoding/decoding,
fixing some edge cases. No new functionality was added in this
commit.

lib/requests_oauthlib/__init__.py

@@ -0,0 +1,19 @@
+import logging
+
+from .oauth1_auth import OAuth1
+from .oauth1_session import OAuth1Session
+from .oauth2_auth import OAuth2
+from .oauth2_session import OAuth2Session, TokenUpdated
+
+__version__ = "1.3.0"
+
+import requests
+
+if requests.__version__ < "2.0.0":
+    msg = (
+        "You are using requests version %s, which is older than "
+        "requests-oauthlib expects, please upgrade to 2.0.0 or later."
+    )
+    raise Warning(msg % requests.__version__)
+
+logging.getLogger("requests_oauthlib").addHandler(logging.NullHandler())