`test_httplib_stdlib` failure on Mono+macOS

[slozier]

The problem in test_httplib_stdlib is more elusive. I see that the certificate chain passed to verify is empty, therefore the verification of a "good" certificate fails, but don't know why it happens only on Mono. Apparently, CPython 3.6 on macOS no longer uses system root certificates (see BPO-29065) and presumably package certifi is the answer, but I could not verify this because I coudn't get pip to work on Mono/macOS. So to investigate it will take more time.

Originally posted by @BCSharp in #1515 (comment)