[2.7] bpo-38216, bpo-36274: Allow subclasses to separately override validation and encoding behavior (GH-16476)

Backporting this change, I observe a couple of things:

1. The _encode_request call is no longer meaningful because the request construction will implicitly encode the request using the default encoding when the format string is used (request = '%s %s %s'...). In order to keep the code as consistent as possible, I decided to include the call as a pass-through. I'd be just as happy to remove it entirely, but I'll leave that up to the reviewer to decide. It's okay that this functionality is disabled on Python 2 because this functionality was mainly around bpo-36274, which was mainly a concern with the transition to Python 3.
2. Because _encode_request is no longer meaningful, neither is the test for it, so I've removed that test. Therefore, the meaningful part of this test is that for bpo-38216, adding a (underscore-protected) hook to customize/disable validation.

(cherry picked from commit 7774d783)
Co-authored-by: Jason R. Coombs <jaraco@jaraco.com>