MAINT: fixes for tox >=4 compatibility

Author: bsipocz

tox.ini

@@ -7,13 +7,12 @@ envlist =
 requires =
     setuptools >= 30.3.0
     pip >= 19.3.1
-    tox-pypi-filter >= 0.12
 isolated_build = true
 
 [testenv]
 
 # Pass through the following environment variables which are needed for the CI
-passenv = HOME WINDIR CI
+passenv = HOME,WINDIR,CI
 
 # Run the tests in a temporary directory to make sure that we don't import
 # astropy from the source tree