Port Python benchmark to pytest-benchmark

[mre]

Our current Python benchmarks are based on ujson's script here.
It's a bit clunky to use and not really idiomatic (redundant code, custom implementation for printing the results as a table).
In addition to that, it's hard to make code changes and see the performance impact, because the measurements fluctuate a lot from run to run.

A modern alternative is pytest-benchmark. Since we already use pytest, it's would fit in well with the rest of our testing pipeline.
It provides powerful output methods for different use-cases:

It can also plot histograms to visualize the error bars as a box-plot.

It would be nice to move to pytest-benchmark as we could compare the measurements after each change using Travis CI.

[wdv4758h]

Not directly related to benchmark, I find out pytest-sugar shows pretty sweat test progress and start using it this year. Just FYI, wonder if you guys have interest 😃

[mre]

@wdv4758h sounds like a cool thing to have. If you like, go ahead and create a PR for it. 👍 🍭

[mre]

Initial work is done by @RSabet in #19.

[mre]

Closing this as #19 is done now. @wdv4758h a pull request for pytest-sugar is appreciated if you find the time. 😃