[WIP] Use nep29 to help with the Python 3.6 -> 3.9 transition

[hmaarrfk]

Description

Migrates builds from python 3.6 to python 3.9 using nep29 to bump minimum version requirements.

Checklist

• Docstrings for all functions
• Gallery example in ./doc/examples (new features only)
• Benchmark in ./benchmarks, if your changes aren't covered by an
  existing benchmark
• Unit tests
• Clean style in the spirit of PEP8

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.

[jni]

Nice, thanks @hmaarrfk! Please let me know if there is anything you'd like to discuss here. It looks to me like Py3.9 will need to be side-loaded rather than using the CI service VMs?

[grlee77]

It looks to me like Py3.9 will need to be side-loaded rather than using the CI service VMs?

As of a week or two ago, both Appveyor and Travis have Python 3.9 available directly. For Appveyor, you might have to select the "Visual Studio 2019" image (https://www.appveyor.com/updates/2020/10/26/)

[hmaarrfk]

It seems osx is timing out. Time to move to azure.????

[grlee77]

It seems osx is timing out. Time to move to azure.????

Given that Travis recently announced that OSX builds on Travis are no longer going to be free, it doesn't seem that we will be able to keep them as is in any case.

There are also new restrictions to 1000 minutes on Linux builds for free accounts. I don't know what our monthly usage is now, but I don't think 1000 minutes gets us very far. I think SciPy is planning to switch existing Travis linux builds to GitHub Actions.

[hmaarrfk]

@jni I hope you aren't waiting on this PR to release the next version.

This infrastructure stuff just likes to grow in scope.

[hmaarrfk]

It seems builds are failing due to missing optional dependencies for python 3.9. i suggest skipping them for now.

I'll try to work on that later. I think we have all the environment variables in place for that.

[soupault]

Minor suggestion on changing the note. Other than that, I am +1 on merging this PR and moving to the next step.

Travis is failing on few builds. Probably related:

# (in `test_thresholding`)
>       pytest.importorskip('dask', reason="dask python library is not installed")
E       TypeError: importorskip() got an unexpected keyword argument 'reason'

[grlee77]

This has become outdated (no Travis CI anymore, already dropped Python 3.6 and added 3.9).

There are a handful of requirement .txt file increments (seaborn, networkx, etc.) that could be proposed in a standalone PR if desired.