DOC: Use date of current commit instead of current date

Author: mgeier

doc/conf.py

@@ -93,6 +93,11 @@
 #today = ''
 # Else, today_fmt is used as the format for a strftime call.
 #today_fmt = '%B %d, %Y'
+try:
+    today = check_output(['git', 'show', '-s', '--format=%ad', '--date=short'])
+    today = today.decode().strip()
+except Exception:
+    today = '<unknown date>'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.