PEP 476: enable HTTPS certificate verification by default (#22417)

Patch by Alex Gaynor with some modifications by me.

--HG--
branch : 3.4
merge 3.4 (#22417)
move idna test domain to pythontest.net

--HG--
branch : 3.2
merge 3.2

--HG--
branch : 3.3
merge 3.3

--HG--
branch : 3.4
merge 3.4
context has been backported so fix versionchanged
test that keyfile can be None

--HG--
branch : 3.4
merge 3.4