Pyperf

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* Add ``Benchmark.update_metadata()`` method
* Warmup samples can now be zero. TextRunner now raises an error if a sample
function returns zero for a sample, except of calibration and warmup samples.

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* Support PyPy
* metadata: add ``mem_max_rss`` and ``python_hash_seed``
* Add :func:`perf.python_implementation` and :func:`perf.python_has_jit`
functions
* In workers, calibration samples are now stored as warmup samples.
* With a JIT (PyPy), the calibration is now done in each worker. The warmup
step can compute more warmup samples if a raw sample is shorter than the
minimum time.
* Warmups of Run objects are now lists of (loops, raw_sample) rather than lists
of samples. This change requires a change in the JSON format.

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* add a new ``slowest`` command
* convert: add ``--extract-metadata=NAME``
* add ``--tracemalloc`` option: use the ``tracemalloc`` module to track
Python memory allocation and get the peak of memory usage in metadata
(``tracemalloc_peak``)
* add ``--track-memory`` option: run a thread reading the memory usage
every millisecond and store the peak as ``mem_peak`` metadata
* ``compare_to``: add ``--group-by-speed`` (``-G``) and ``--min-speed`` options
* metadata: add ``runnable_threads``
* Fix issues on ppc64le Power8

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* Add start/end dates and duration to the ``stats`` command
* Fix the program name: ``pyperf``, not ``pybench``!
* Fix the ``-b`` command line option of show/stats/... commands
* Fix metadata: ``load_avg_1min=0.0`` is valid!

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* Fix the ``--append`` command line option

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* Add a new ``pybench`` program, similar to ``python3 -m perf``
* Most perf CLI commands now support multiple files and support benchmark
suites.
* Add a new ``dump`` command to the perf CLI and a ``--dump`` option to
the TextRunner CLI
* ``convert`` command: add ``--indent`` and ``--remove-warmups`` options
* replace ``--json`` option with ``-o/--output``
* New metadata:

- cpu_config
- cpu_freq
- cpu_temp
- load_avg_1min

Changes:

* New :func:`add_runs` function.
* Once again, rewrite Run and Benchmark API. Benchmark name is now optional.
* New :class:`Run` class: it now stores normalized samples rather than raw
samples
* Metadata are now stored in Run, no more in Benchmark.
Benchmark.get_metadata() return metadata common to all runs.
* Metadata become typed (can have a different type than string), the
new :class:`Metadata` class formats them.