Update maximum Astropy version
Update maximum Astropy version

We are purposedly _not_ adding checks in the CI services
to test poliastro against older versions of Astropy.
It is _meant_ to work, but for the moment we will not
dedicate the energy to properly assess that.

anhiga/poliastro,newlawrence/poliastro,anhiga/poliastro,poliastro/poliastro,newlawrence/poliastro,newlawrence/poliastro,Juanlu001/poliastro,anhiga/poliastro,Juanlu001/poliastro,Juanlu001/poliastro