Narrow down import constraint to astropy.units

.importlinter

@@ -1,13 +1,15 @@
 [importlinter]
-root_package = poliastro
+# Include Astropy to properly analyze external submodules,
+# see https://github.com/seddonym/import-linter/issues/111
+root_packages =
+    poliastro
+    astropy
 include_external_packages=True
 
 [importlinter:contract:1]
 name=poliastro.core does not import astropy.units
 type=forbidden
 source_modules=
     poliastro.core
-# astropy.units is not supported,
-# see https://github.com/seddonym/import-linter/issues/111
 forbidden_modules=
-    astropy
+    astropy.units