The no_proxy environment variable works in python as a case
sensitive suffix check.

Curl handles this variable as a case insensitive hostname check.

Case sensitivity appears to be in conflict with the DNS Case
Insensitivity RFC (https://tools.ietf.org/html/rfc4343).

While the suffix check is documented
(https://docs.python.org/3/library/urllib.request.html), this
seems to be problematic and inconsistent with other tools on the
system.

I believe the ideal solution would be to have proxy_bypass be a
method of ProxyHandler so that it can be overridden without
dependence on undocumented methods. This would also allow
for the requested behavior to be added without breaking backwards
compatibility.