fix problem with python version

[tduberne]

numpy failed to compile using the "latest" version from python:3
using python:3.6 fixes the issue

issue remains if using python:3.7

I did this fix for myself, and thought you might be interested in merging it back on your side (or find a solution you find more suitable ;-))

[plattenschieber]

Especially the makefile is useful! Thx @tduberne Is numpy still not compiling under 3.7?

[jgoerner]

Thanks @tduberne for your support - the Makefile is indeed a helpful tool
As asked by @plattenschieber, do you still encouter problems with the numpy compilation?

I will take a look at your PR as well as #2 and merge them ASAP.

[jgoerner]

Hey @tduberne

Sorry for coming back so lately. As I seem to already changed the Python version in a previous commit that topic should be fixed - however I really like your Makefile approach and would be happy to include it in the codebase.

Since you came up with that idea (and deserve the kudos) I would appreciate if you might open another PR based on the newest version of this repo - including the Makefile.

Would that be fine with you?

[tduberne]

Sorry for the extremely late reply, this went buried in a massive amount of GitHub notifications...

No problem for the PR, I will do it