Fix extra links to follow latest format

The way it is done currently has been deprecated for a long time and support will be removed in version 8 of sphinx.

This merge request also removes the handling of extra-intersphinx as it is an unused feature.

The handling of the file is done in such a way that it allows to included external dependencies when used only for documentation purpose however this is a limited list as shown in the implementation.

conda/meta.yaml

@@ -65,11 +65,11 @@ test:
     - conda inspect objects -p $PREFIX {{ name }}  # [osx]
   requires:
     - bob.extension
-    - ddt
-    - nose
-    - coverage
-    - sphinx
-    - sphinx_rtd_theme
+    - ddt {{ ddt }}
+    - nose {{ nose }}
+    - coverage {{ coverage }}
+    - sphinx {{ sphinx }}
+    - sphinx_rtd_theme {{ sphinx_rtd_theme }}
 
 about:
   home: https://www.idiap.ch/software/beat/