[WIP] Use PEP 517 Build Isolation

[Cadair]

Description

This is following on from a discussion in #4261

This works for me currently when installing from a checkout in a completely clean venv. What I am unclear on at this point is how the C files are generated for the sdist, I am proposing that we remove that behaviour and have people generate the C files during build (with a pinned known version of Cython).

For reviewers

• Check that the PR title is short, concise, and will make sense 1 year
  later.
• Check that new functions are imported in corresponding __init__.py.
• Check that new features, API changes, and deprecations are mentioned in
  doc/release/release_dev.rst.
• Consider backporting the PR with @meeseeksdev backport to v0.14.x

[Cadair]

I have pinned numpy 1.14.5 as it's the first numpy version with py37 wheels on pypi. I have no idea what a good Cython pin is, there is no reason why we couldn't go very new here if we needed to.

[hmaarrfk]

This works for python 3.8?

[grlee77]

I think the strategy is generally to choose the minimum NumPy that is both compatible with scikit-image and for which wheels exist. That is why SciPy has all of the following cases:

https://github.com/scipy/scipy/blob/6644a54ff862e4d90d7f9bc43ece9b0b1f681861/pyproject.toml#L5-L13

[grlee77]

Also, I think Cython >= 0.29.13 is the minimum compatible with Python 3.8.

[Cadair]

yes:

$ python --version
Python 3.8.0
$ pip freeze
pkg-resources==0.0.0
$ pip install .

worked fine.

I assume it's compiled numpy and cython from source.

[Cadair]

I know that this build isolation used to rely on wheels existing, but it seems from this experiment that that is no longer the case. I wonder if I can find a pip reference for that somewhere.

[Cadair]

This feels very weird to me, but I have left it alone.