Drop urllib3 in favor of public gist (#1182)

Drop urllib3 in favor of public gist (#1182)

* Drop urllib3 in favor of public gist

* Drop urllib3 coverage omit

* Drop recommendation to use urllib3 transport during Requests migration

* Add urllib3-transport to 3p pkgs

* Drop urllib3 from dependencies list in README / docs home page

Co-authored-by: Tom Christie <tom@tomchristie.com>