Enable Plausible for RTD but not RTD previews (#1751)

Author: hugovk

_templates/page.html

@@ -2,7 +2,7 @@
 
 {% block extrahead %}
 {{ super() }}
-{% if not READTHEDOCS %}
+{% if not is_deployment_preview %}
 <script defer data-domain="devguide.python.org" src="https://analytics.python.org/js/script.outbound-links.js"></script>
 {% endif %}
 {% endblock %}

conf.py

@@ -52,8 +52,12 @@
 # Set to '' to prevent appending "documentation" to the site title
 html_title = ""
 
+# Deployment preview information
+# (See .readthedocs.yaml and https://docs.readthedocs.io/en/stable/reference/environment-variables.html)
+is_deployment_preview = os.getenv("READTHEDOCS_VERSION_TYPE") == "external"
+
 html_context = {
-    "READTHEDOCS": bool(os.getenv("READTHEDOCS")),
+    "is_deployment_preview": is_deployment_preview,
 }
 
 linkcheck_allowed_redirects = {