Support both Python 2 and Python 3 in tests/attr.py

The use of "except as" syntax implies the minimum supported Python2 version is
now v2.6

Committer testing:

  $ make -C tools/perf PYTHON3=python install-bin

Before:

  # perf test attr
  16: Setup struct perf_event_attr                          : FAILED!
  48: Synthesize attr update                                : Ok
  [root@quaco ~]# perf test -v attr
  16: Setup struct perf_event_attr                          :
  --- start ---
  test child forked, pid 3121
    File "/home/acme/libexec/perf-core/tests/attr.py", line 324
      except Unsup, obj:
                ^
  SyntaxError: invalid syntax
  test child finished with -1
  ---- end ----
  Setup struct perf_event_attr: FAILED!
  48: Synthesize attr update                                :
  --- start ---
  test child forked, pid 3124
  test child finished with 0
  ---- end ----
  Synthesize attr update: Ok
  #

After:

   # perf test attr
  16: Setup struct perf_event_attr                          : Ok
  48: Synthesize attr update                                : Ok
  #
Signed-off-by: Tony Jones <tonyj@suse.de>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Seeteena Thoufeek <s1seetee@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20190124005229.16146-7-tonyj@suse.deSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>