Add packaging as dependency for tests and doc
Add packaging as dependency for tests and doc

Ref https://github.com/astropy/astropy/pull/11091#pullrequestreview-630266621
packaging is used in tests and the Sphinx config so we should list it
explicitly as a dependency, even if it's already a dependency for pytest
and Sphinx.

StuartLittlefair/astropy,pllim/astropy,larrybradley/astropy,lpsinger/astropy,mhvk/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,saimn/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy,mhvk/astropy,StuartLittlefair/astropy,pllim/astropy,saimn/astropy,saimn/astropy,lpsinger/astropy,StuartLittlefair/astropy,larrybradley/astropy,pllim/astropy,larrybradley/astropy,astropy/astropy,mhvk/astropy,pllim/astropy,astropy/astropy,aleksandr-bakanov/astropy,aleksandr-bakanov/astropy,mhvk/astropy,astropy/astropy,saimn/astropy,larrybradley/astropy,StuartLittlefair/astropy,lpsinger/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,pllim/astropy,aleksandr-bakanov/astropy,lpsinger/astropy