DEP: defusedxml is an optional code rather than test dependency

Author: bsipocz

README.rst

@@ -57,6 +57,7 @@ The following packages are optional dependencies and are required for the
 full functionality:
 
  * pillow
+ * defusedxml
 
 For running the tests, and building the documentation, the following
 infrastructure packages are required:

setup.cfg

@@ -70,11 +70,11 @@ python_requires = >=3.8
 [options.extras_require]
 all =
     pillow
+    defusedxml
 test =
     pytest-doctestplus>=0.13
     pytest-astropy
     requests-mock
-    defusedxml
 docs =
     sphinx-astropy