pythongh-121871: Allow overriding docs build date with SOURCE_DATE_EP…

…OCH (pythonGH-121872)

(cherry picked from commit 941b3b7)

Co-authored-by: Bernhard M. Wiedemann <bwiedemann@suse.de>

Doc/conf.py

@@ -347,7 +347,8 @@
 }
 
 # This 'Last updated on:' timestamp is inserted at the bottom of every page.
-html_last_updated_fmt = time.strftime('%b %d, %Y (%H:%M UTC)', time.gmtime())
+html_time = int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))
+html_last_updated_fmt = time.strftime('%b %d, %Y (%H:%M UTC)', time.gmtime(html_time))
 
 # Path to find HTML templates.
 templates_path = ['tools/templates']