www/mitmproxy: Prepare for py-openssl 19.0.0 and unbreak at runtime

Relax the version ranges for security/py-openssl to avoid breakage at
runtime when release 19.0.0 lands into the Ports tree.

Also unbreak the port as it's already broken at runtime because the given
requirements for security/py-cryptography are too narrow.

PR: 239540
Approved by: Hung-Yi Chen <gaod@hychen.org> (maintainer)
MFH: 2019Q3 (runtime fix)