Fix Pow._eval_is_rational for integer exponents

closes #8598
UnXFAIL some diffgeom tests in the test_args.py
Abs evaluates as integer/rational when needed
Merge pull request #8599 from pelegm/abs_assumptions

Abs evaluates as integer/rational when needed
Merge pull request #8600 from skirpichev/8598-fix

Fix Pow._eval_is_rational for integer exponents