Refactor RequestedAuthnContext

[peppelinux]

following this IdentityPython/pysaml2#807
we'll wait for the next release of pysaml2 to refactor by our side this feature, that actually is based on this
#289

[peppelinux]

@c00kiemon5ter @pandafy ^

[peppelinux]

Closed by
https://github.com/IdentityPython/djangosaml2/releases/tag/v1.3.5