scheduler and worker fail in PyPy3

Python version: Python 3.5.3 (89428233efed, Apr 12 2018, 16:18:00)
Platform: pypy3-5.10
OS: Fedora 28 5.0.16-100.fc28.x86_64

Calling dask-scheduler or dask-worker fails due to GC tuning from https://github.com/dask/distributed/issues/1653.

```
[root@localhost ~]$ dask-scheduler
Traceback (most recent call last):
File "/usr/lib64/pypy3-5.10/bin/dask-scheduler", line 10, in
sys.exit(go())
File "/usr/lib64/pypy3-5.10/site-packages/distributed/cli/dask_scheduler.py", line 245, in go
main()
File "/usr/lib64/pypy3-5.10/site-packages/click/core.py", line 764, in __call__
return self.main(*args, **kwargs)
File "/usr/lib64/pypy3-5.10/site-packages/click/core.py", line 717, in main
rv = self.invoke(ctx)
File "/usr/lib64/pypy3-5.10/site-packages/click/core.py", line 956, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/lib64/pypy3-5.10/site-packages/click/core.py", line 555, in invoke
return callback(*args, **kwargs)
File "/usr/lib64/pypy3-5.10/site-packages/distributed/cli/dask_scheduler.py", line 141, in main
g0, g1, g2 = gc.get_threshold() # https://github.com/dask/distributed/issues/1653
```

Commenting out these lines solves the problem. Maybe include a check using `platform.python_implementation()` before attempting to tune the GC.