gdb/testsuite: analyze-racy-logs.py cleanup

 - Add type annotations
 - Use a raw string in one spot (where we call re.sub), to avoid an
   "invalid escape sequence" warning.
 - Remove unused "os" import.

Change-Id: I0149cbb73ad2b05431f032fa9d9530282cb01e90
Reviewed-By: Guinevere Larsen <blarsen@redhat.com>