perf python: Make twatch.py work with both python2 and python3

Will be used to test patches allowing to build perf with python3, so
that we make sure that we can build with both versions.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jaroslav Škarvada <jskarvad@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-c2ynv0ozr3eifzsyit6qgh3h@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>