Updated conf.py to fix CI issue

It seems like `_static` dir is not found.
Attempt a fix according to:
readthedocs/readthedocs.org#1776 (comment)

Author: realcr

docs/source/conf.py

@@ -49,4 +49,5 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = ['_static']
+# html_static_path = ['_static']
+html_static_path = []