Revert pysaml2 requirement version back to 2.2.0
Revert pysaml2 requirement version back to 2.2.0

It looks like 2.3.0 broke something, at least in my app.

MiguelSR/djangosaml2,knaperek/djangosaml2,WebSpider/djangosaml2,writepython/djangosaml2,BetterWorks/djangosaml2,BetterWorks/djangosaml2,knaperek/djangosaml2,WebSpider/djangosaml2,MiguelSR/djangosaml2,writepython/djangosaml2