Update intersphinx inventory (#1893)

Resolve the following warnings when `make clean html`.

```
parsing bibtex file /torchaudio/docs/source/refs.bib... parsed 26 entries
loading intersphinx inventory from https://docs.python.org/objects.inv...
loading intersphinx inventory from https://docs.scipy.org/doc/numpy/objects.inv...
loading intersphinx inventory from https://pytorch.org/docs/stable/objects.inv...
intersphinx inventory has moved: https://docs.python.org/objects.inv -> https://docs.python.org/3/objects.inv
intersphinx inventory has moved: https://docs.scipy.org/doc/numpy/objects.inv -> https://numpy.org/doc/stable/objects.inv
```

docs/source/conf.py

@@ -199,8 +199,8 @@ texinfo_documents = [
 
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {
-    'python': ('https://docs.python.org/', None),
-    'numpy': ('https://docs.scipy.org/doc/numpy/', None),
+    'python': ('https://docs.python.org/3/', None),
+    'numpy': ('https://numpy.org/doc/stable/', None),
     'torch': ('https://pytorch.org/docs/stable/', None),
 }