Numba: Depend on enum34 using PEP 508 environment markers

[dsevero]

Reporting a bug

• [x] I am using the latest released version of Numba (most recent is visible in
  the change log (https://github.com/numba/numba/blob/master/CHANGE_LOG).
• [x] I have included below a minimal working reproducer (if you are unsure how
  to write one see http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports).

This is a straight up copy of https://github.com/numba/llvmlite/pull/483

Looks like the offending lines are https://github.com/numba/numba/blob/master/setup.py#L309-L312 as mentioned by @esc

[stuartarchibald]

Thanks for the report. Fixed in #4104

[dsevero]

llvmlite had the same issue and has been fixed at the master branch, but a new release has not been made. Hence, this issue alone will not suffice. We need a new release of llvmlite.

Right?

[stuartarchibald]

The core team are in the process of producing release candidates as it's the end of a development cycle, rest assured that they will appear shortly. Numba takes longer to release as the build farm has to churn through literally hundreds of combinations, but they'll appear in the next few days!