docs: link to python 3.8 docs

because py38 is our minimum requirement.

src/borg/archiver.py

@@ -2452,11 +2452,11 @@ class Archiver:
 
         {now}
             The current local date and time, by default in ISO-8601 format.
-            You can also supply your own `format string <https://docs.python.org/3.5/library/datetime.html#strftime-and-strptime-behavior>`_, e.g. {now:%Y-%m-%d_%H:%M:%S}
+            You can also supply your own `format string <https://docs.python.org/3.8/library/datetime.html#strftime-and-strptime-behavior>`_, e.g. {now:%Y-%m-%d_%H:%M:%S}
 
         {utcnow}
             The current UTC date and time, by default in ISO-8601 format.
-            You can also supply your own `format string <https://docs.python.org/3.5/library/datetime.html#strftime-and-strptime-behavior>`_, e.g. {utcnow:%Y-%m-%d_%H:%M:%S}
+            You can also supply your own `format string <https://docs.python.org/3.8/library/datetime.html#strftime-and-strptime-behavior>`_, e.g. {utcnow:%Y-%m-%d_%H:%M:%S}
 
         {user}
             The user name (or UID, if no name is available) of the user running borg.