CI: don't use pyproject.toml in the CI builds

[pv]

Pass on --no-build-isolation to pip, so that it does not break the numpy
version fixing etc. careful environment setup in the CI.

In principle we might want to check the build isolation stuff works in the CI,
but that's for another day.

Make the python 3.5 wheel build use build isolation, the rest of the builds don't.

Trying to use build isolation gives the error (https://travis-ci.org/scipy/scipy/jobs/366522826)

pip wheel
Processing /home/travis/build/scipy/scipy
  Could not find a version that satisfies the requirement numpy==1.8.2 (from versions: 1.9.0, 1.9.1, 1.9.2, 1.9.3, 1.10.0, 1.10.1, 1.10.2, 1.10.3, 1.10.4, 1.11.0, 1.11.1rc1, 1.11.1, 1.11.2rc1, 1.11.2, 1.11.3, 1.12.0b1, 1.12.0rc1, 1.12.0rc2, 1.12.0, 1.12.1rc1, 1.12.1, 1.13.0rc1, 1.13.0rc2, 1.13.0, 1.13.1, 1.13.3, 1.14.0rc1, 1.14.0, 1.14.1, 1.14.2)
No matching distribution found for numpy==1.8.2

Fixing that probably is for another day...

[pv]

I'm going to remove pyproject.toml from scipy 1.1.0, this stuff is untested and I think not fit for release at the moment.

Too late for that, it was already in scipy 1.0.0 so we'll just have to live with it.
The issue with the Windows failures is that on Windows, the numpy version to use for releases is different as it's based on the availability of wheels.

Apparently pip also considers only wheels for deciding "what versions are available", which also breaks the travis builds as we want to build against numpy 1.8.2 that has no wheels for py3.5.

[pv]

Merging to fix the CI