TST: Install astropy-dev properly

tox.ini

@@ -12,7 +12,7 @@ isolated_build = true
 # Suppress display of matplotlib plots generated during docs build
 setenv =
     MPLBACKEND=agg
-    devdeps: PIP_EXTRA_INDEX_URL = https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
+    devdeps: PIP_EXTRA_INDEX_URL = https://pypi.anaconda.org/astropy/simple https://pypi.anaconda.org/scientific-python-nightly-wheels/simple
 
 # Pass through the following environment variables which may be needed for the CI
 passenv = HOME,WINDIR,LC_ALL,LC_CTYPE,CC,CI
@@ -53,6 +53,7 @@ deps =
 
     devdeps: numpy>=0.0.dev0
     devdeps: scipy>=0.0.dev0
+    devdeps: astropy>=0.0.dev0
     devdeps: git+https://github.com/spacetelescope/gwcs.git
     devdeps: git+https://github.com/asdf-format/asdf.git
     devdeps: git+https://github.com/astropy/asdf-astropy.git
@@ -64,7 +65,6 @@ extras =
     !oldestdeps: jwst
 
 commands =
-    devdeps: pip install -i https://pypi.anaconda.org/astropy/simple astropy --pre --upgrade
     pip freeze
     !cov: pytest --pyargs specutils {toxinidir}/docs {posargs}
     cov: pytest --pyargs specutils {toxinidir}/docs --cov specutils --cov-config={toxinidir}/setup.cfg {posargs}