Make kerberos dependency optional

[gsemet]

Kerberos package is a mess to install on some environment such as alpine or on Windows (it requires VC++14, ...). I do not need it, so I wonder if this dependency cannot be made optional ?

The Kerberos changes since 1.13.31 have also broken the pip3 install of the package on Ubuntu and Debian Linux.

[offa]

I've opened a work-in-progress PR to address this. There are still some issues left, so helping hands and comments are welcome.

[gonchik]

I would say now everything works well :)
Thanks @offa for your effort