related to #472

urllib3 could for example use create_default_context in create_urllib3_context if it is available

also i'd move certificate path logic into create_urllib3_context (why is it separate, anyway?)

i'd also propose to auto-call context.set_default_verify_paths() if available and cert_reqs is other than CERT_NONE

i can send a PR with all or some of this if you want it; just wanted to ask in advance if this is something desirable