conf: Add sphinx_rtd_theme to extensions, remove env based import

The RTD build failed with "NameError: name 'html_theme' is notdefined".

Remove the conditional import and align to what Spicy and the
sphinx_rtd_theme docs [1] indicate should be done.

[1] https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html

conf.py

@@ -28,6 +28,7 @@
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
 extensions += ['zeek', 'sphinx.ext.todo', 'zeek_pygments', 'literal-emph']
 extensions += ['sphinx.ext.extlinks']
+extensions += ['sphinx_rtd_theme']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
@@ -141,15 +142,7 @@
 
 # -- Options for HTML output ---------------------------------------------------
 
-# The theme to use for HTML and HTML Help pages.  See the documentation for
-# a list of builtin themes.
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-
-if not on_rtd:
-    # only import and set the theme if we're building docs locally
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = 'sphinx_rtd_theme'
 
 html_last_updated_fmt = '%B %d, %Y'