Expand the range of supported setuptools.

[jsirois]

We now just exclude the island where setuptools ran a disruptive
de-vendoring experiment.

[jsirois]

I manually tested pex --pre with setuptools versions inside and outside the excluded range, only getting failures in the excluded range. I used https://github.com/pypa/setuptools/blob/master/CHANGES.rst to find the exclusion range in the first place.

The motivation here is to let Pants use a newer version of setuptools so that it can run under python 3.7. See: pantsbuild/pants#6363

[kwlzn]

lgtm

[kwlzn]

the use and explicit del of __exclusions struck me as odd here. any reason to not keep the exclusions around as a constant (like SETUPTOOLS_EXCLUSIONS) - or just elide and hardcode in the string literal?

[jsirois]

Agreed - I'll inline. This was half remembering wickmanisms and half wishing for assignment blocks:

x = block:
  foo
  # NB: bar is very very worth exposition
  bar

  'baz'

assert x == 'baz'