[MRG] Cleanup minimal build and add separate build for pep8

[rflamary]

Types of changes

• cleanupothe build_minimal that had numpy, scipy cython version forced and was failing
• add a separate pep8 build so taht all teh otehr test can still run in pep8 fails

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and context / Related issue

The build was failing in all the PR becausqe of that.

The pep8 was a pain when a new commit was necessary in order to get tye output of the tests when pep8 failure

How has this been tested (if it applies)

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document.
• All tests passed, and additional code has been covered with new tests.

[rflamary]

I'm a bit puzzled by this. The minimal build (during the tests) fails most of the time with just a coment that it was too long.

If you rerun it several times sometimes it passes. This an an awfull bug that i cannot reproduce on my machine.

It usually gets stuck after
test/test_ot.py::test_emd_empty PASSED [ 55%]

so during test_emd2_multi

this one has been a pain and i'm thinking about rewriting the multi-proc implementation but it will take some time.