remove deprecated get_html_theme()

docs/conf.py

@@ -102,17 +102,7 @@
 #
 on_rtd = os.environ.get("READTHEDOCS", None) == "True"
 
-if not on_rtd:  # only import and set the theme if we're building docs locally
-    try:
-        import sphinx_rtd_theme
-
-        html_theme = "sphinx_rtd_theme"
-        html_theme_path = [sphinx_rtd_theme.get_html_theme_path(), "."]
-    except:
-        html_theme = "default"
-        html_theme_path = ["."]
-else:
-    html_theme_path = ["."]
+html_theme = "sphinx_rtd_theme"
 
 # 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,