CI: testing with pytest dev version to smoke out issues sooner rather than later

Author: bsipocz

tox.ini

@@ -34,6 +34,7 @@ deps =
     devdeps: astropy>=0.0.dev0
     devdeps: pyerfa>=0.0.dev0
     devdeps: git+https://github.com/astropy/pyvo.git#egg=pyvo
+    devdeps: git+https://github.com/pytest-dev/pytest.git#egg=pytest
 
 # mpl while not a dependency, it's required for the tests, and would pull up a newer numpy version if not pinned.
 # And pillow should be pinned as well, otherwise a too new version is pulled that is not compatible with old np.