Remove hard certifi dependency and document.
Link to Mozilla trust store.
Merge pull request #2203 from Lukasa/remove_certifi

Remove hard certifi dependency and document.
Fix #2207

@Lukasa wrote the fix in #2207
Request->Response.
Merge pull request #2213 from kennethreitz/req-rsp

Request->Response.
Merge pull request #2210 from sigmavirus24/bug/2207

Fix #2207
requests v2.4.1
mils