Port to python3

[traumschule]

​https://docs.python.org/3/howto/pyporting.html
​https://docs.python.org/2/library/2to3.html#to3-reference

[irl]

Starting point, the output from pylint3 --py3k:

************* Module onionperf.measurement
onionperf/measurement.py:7:0: W1618: import missing `from __future__ import absolute_import` (no-absolute-import)
onionperf/measurement.py:7:0: W1648: Module moved in Python 3 (bad-python3-import)
************* Module onionperf.visualization
onionperf/visualization.py:7:0: W1618: import missing `from __future__ import absolute_import` (no-absolute-import)
onionperf/visualization.py:49:0: W1623: Assigning to a class's __metaclass__ attribute (metaclass-assignment)
onionperf/visualization.py:545:13: W1619: division w/o __future__ statement (old-division)
onionperf/visualization.py:549:18: W1619: division w/o __future__ statement (old-division)
onionperf/visualization.py:556:8: W1619: division w/o __future__ statement (old-division)
onionperf/visualization.py:558:13: W1613: xrange built-in referenced (xrange-builtin)
onionperf/visualization.py:558:24: W1633: round built-in referenced (round-builtin)
************* Module onionperf.monitor
onionperf/monitor.py:7:0: W1618: import missing `from __future__ import absolute_import` (no-absolute-import)
************* Module onionperf.model
onionperf/model.py:7:0: W1618: import missing `from __future__ import absolute_import` (no-absolute-import)
onionperf/model.py:8:0: W1648: Module moved in Python 3 (bad-python3-import)
onionperf/model.py:11:0: W1623: Assigning to a class's __metaclass__ attribute (metaclass-assignment)
onionperf/model.py:45:0: W1623: Assigning to a class's __metaclass__ attribute (metaclass-assignment)
************* Module onionperf.util
onionperf/util.py:7:0: W1618: import missing `from __future__ import absolute_import` (no-absolute-import)
onionperf/util.py:116:11: W1658: Accessing a removed attribute on the urllib module (deprecated-urllib-function)
onionperf/util.py:116:11: W1658: Accessing a removed attribute on the urllib module (deprecated-urllib-function)
onionperf/util.py:151:4: W1653: next method defined (next-method-defined)
onionperf/util.py:155:15: W1622: Called a next() method on an object (next-method-called)
onionperf/util.py:179:0: W1623: Assigning to a class's __metaclass__ attribute (metaclass-assignment)

[irl]

This issue is now being tracked at https://trac.torproject.org/projects/tor/ticket/29367