include tests for multiple sympy>=0.7.2 versions
add sympy0.7.3 to allow_failures
sympy.0.7.3 incompatability changes:
    sympy.Ylm -> sympy.Ynm
    sympy.Expr.as_coeff_terms -> sympy.Expr.as_coeff_mul
    sympy.Symbol('x', dummy=True) -> sympy.Dummy('x')
update Solve tests reflecting sympy.solve improvements