re-introducing 'numpy' as a mock import

...to cope with not installing the same anymore from build_docs.sh

Author: shyamblast

docs/conf.py

@@ -172,7 +172,7 @@
 # -- Autodoc options ---------------------------------------------------------
 autodoc_member_order = 'groupwise'
 autodoc_mock_imports = [
-    #'numpy',
+    'numpy',
     'scipy',
     'soundfile',
     'audioread',