Implement GSSAPI/Kerberos authentication for requests

Currently has two issues which may be related:
- doesn't work with redirects, we try to authenticate the server a second
  time with a fully completed kerberos context.
- 403 responses result in the wrong response object being returned, but
  this is also true for http digest authentication due to a bug in hook
  handling in general.
Added needed cast for when POSTing data and files (Issue #661)

Co-Authored By: Timnit Gebru <tgebru@gmail.com>
Co-Authored By: Sarah Gonzalez <smar.gonz@gmail.com>
Co-Authored By: Leila Muhtasib <muhtasib@gmail.com>
Make OAuth path hack platform independent.
Comment typo fix and move newline.
Appengine workaround
Merge pull request #739 from saevarom/develop

Appengine workaround
Merge pull request #736 from Lukasa/develop

Make OAuth path hack platform independent.
urllib update and junks
Merge pull request #729 from vickimo/issue_661

Added needed cast for when POSTing data and files (Issue #661)
Merge pull request #647 from mkomitee/kerberos

Implement GSSAPI/Kerberos authentication for requests
ordered dict
Merge remote-tracking branch 'origin/develop' into develop
changes
Solves #722
disable tricky tests
v0.13.4
v0.13.5
backport upstream pull request #957 but beware of issue #1051