assorted minor features and bugfixes

• use Python3 by default now.
• various small bugfixes:
  • fix error if a binary build artifact has zero file size
  • fix symlink target in package update links (symlink to maintenance
    incidents).
• new --no-urlopen-wrapper command line option to disable the HTTP connection
  reuse hack. This is helpful when working against build.suse.de, where
  two-factor-authentication is now required and the hack doesn't work with
  that anymore.
• new --no-bin-cache command line option to disable caching of large binary files.
• new --use-logfile command line option to write errors and debugging
  information to file. This is helpful when oscfs is running as a daemon and
  stdout/stderr are discarded.
• implements transparent retry if the OBS server responds with HTTP status 503
  (service unavailable).