fix tests when using Python 3.9a6
fix tests when using Python 3.9a6

In Python 3.9a6 integer values for future flags were changed to prevent
collision with compiler flags. We need to retrieve these at runtime so
the test suite works with Python <= 3.8 as well as Python 3.9.

mitsuhiko/babel,python-babel/babel,mitsuhiko/babel,mitsuhiko/babel,python-babel/babel