docs: update primer for Variables replacing State, Python 3 and current code
docs: update primer for Variables replacing State, Python 3 and current code

rc/sfepy,vlukes/sfepy,BubuLK/sfepy,vlukes/sfepy,BubuLK/sfepy,sfepy/sfepy,sfepy/sfepy,sfepy/sfepy,BubuLK/sfepy,rc/sfepy,vlukes/sfepy,rc/sfepy