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/compliance_fixes/__init__.py

@@ -0,0 +1,10 @@
+from __future__ import absolute_import
+
+from .facebook import facebook_compliance_fix
+from .fitbit import fitbit_compliance_fix
+from .linkedin import linkedin_compliance_fix
+from .slack import slack_compliance_fix
+from .instagram import instagram_compliance_fix
+from .mailchimp import mailchimp_compliance_fix
+from .weibo import weibo_compliance_fix
+from .plentymarkets import plentymarkets_compliance_fix