No rbgen on Fedora because Python 3 only

[holzingk]

Fails to build on Fedora because it ships without Python 2.
This line needs to be changed probably:
https://github.com/sysrepo/libredblack/blob/master/configure#L2959

[jktjkt]

It's not just the TabError, there's also the exec statement on Python 2, which Python 3 does not have. An appropriate fix is passing --without-rbgen to ./configure because sysrepo does not use rbgen at all.