Specify language as 'en' instead of None (#1059)

Needed for Sphinx 5 compatibility. Should fix the following warning occurring in doc builds.

```
Invalid configuration value found: 'language = None'. Update your configuration to a valid langauge code. Falling back to 'en' (English).
```

xref: sphinx-doc/sphinx#10481

Authors:
  - https://github.com/jakirkham
  - GALI PREM SAGAR (https://github.com/galipremsagar)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #1059

python/docs/conf.py

@@ -78,7 +78,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = "en"
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.