Proxy support

[christophvw]

As it looks like that requests-kerberos is dead it might be a good idea to adapt this patch to get proxy support here:
requests/requests-kerberos#148

When installing requests-gssapi I noticed the following:
Requirement already satisfied: urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1

To get proper proxy support urlib3 >= 1.26 will be required.

[christophvw]

#36

[frozencemetery]

Note that requests-gssapi doesn't currently have a dependency on urllib3 directly. If it's important we could add one.

[christophvw]

It looks like my patch works with http only, https failed in my tests. :/