Do not pin astropy bugfix version

Author: pllim

tox.ini

@@ -34,7 +34,7 @@ deps =
 # minimum numpy to 1.15. mpl while not a dependency, it's required for the
 # tests, and would pull up a newer numpy version if not pinned.
 
-    oldestdeps: astropy==4.0.6
+    oldestdeps: astropy==4.0
     oldestdeps: numpy==1.16
     oldestdeps: matplotlib==3.3.*
     cov: codecov