Support x.y.z.w/prefix form in no_proxy

CIDR format is usually used in no_proxy environment varilable.
Now requests modules supports only domain suffix form (.mydomain.example.net) for no_proxy.
I believe it is useful to support CIDR format in no_proxy.

After this commit, we can use no_proxy such as "no_proxy=127.0.0.0/8,192.168.0.0/16,10.0.0.0/8,.mydomain.example.net"

This patch uses 'netaddr' module. I am not sure using a third party module is acceptable in requests module policy.

In addition, I cannot find unit tests in the repo.

Any feedback will be appreciated.
Thanks,
Akihiro