[pypy-issue] Issue #2733: pypy3 slowdown for json_dumps benchmark (pypy/pypy)

[mattip]

New issue 2733: pypy3 slowdown for json_dumps benchmark
https://bitbucket.org/pypy/pypy/issues/2733/pypy3-slowdown-for-json_dumps-benchmark

mattip:

Using the cpython performance set of benchmarks, pypy3-v5.10.1 is very slow as if the JIT is off.
```
#setup
python -mpip install performance; python -mperformance venv create
pypy2 -mpip install performance; pypy2 -mperformance venv create
pypy3 -mpip install performance; pypy3 -mperformance venv create
# runs the bnechmark
python -mperformance run -b json_dumps venv venv/pypy3* 
python -mperformance run -b json_dumps venv venv/pypy2* 
python -mperformance run -b json_dumps venv venv/cpython* 
```
Gives me ~5ms for pypy2, ~17 ms for cpython2, ~50 ms for pypy3