fixup! amari.xlog: Implement log rotation

Xavier reports that str|None is supported only by python ≥ 3.10, while
we still should care to support at least 3.9 - e.g. SlapOS uses it by
default as well as Debian 11.

Let's not delve deep into typing game. If we cannot express things
easily we can omit the type completely or express it in comments.

/reported-by @xavier_thompson at kirr/xlte!5 (comment 182930)

amari/xlog.py

@@ -462,7 +462,7 @@ _xmsg("x.drb_stats", drb._x_stats_srv, "retrieve statistics about data radio bea
 
 # _openwriter opens destination log file for writing.
 # the file is configured to be logrotated according to rotatespec.
-def _openwriter(path: str, rotatespec: str|None) -> IWriter:
+def _openwriter(path: str, rotatespec) -> IWriter:
     if rotatespec is None:
         return _PlainWriter(path)