Fix sphinx import in docs extension (#1376)

The qiskit-experiments Sphinx extension was importing Sphinx from an
internal location that was removed in 7.2. Here the import is moved to
the main locaiont for the Sphinx app.

docs/_ext/custom_styles/option_parser.py

@@ -21,7 +21,8 @@
 from typing import Type, Optional
 
 import numpy as np
-from sphinx.ext.autodoc import Sphinx, Options as SphinxOptions
+from sphinx.application import Sphinx
+from sphinx.ext.autodoc import Options as SphinxOptions
 from sphinx.ext.napoleon import Config as NapoleonConfig
 from sphinx.ext.napoleon import GoogleDocstring