DOC: Add Python 2 statement to README, reference maintenance branch in CONTRIBUTING

[effigies]

Summary

Ref #3113.

List of changes proposed in this PR (pull-request)

• Add a Python 2 statement to README
• Mention maintenance branch in CONTRIBUTING

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.

[effigies]

This could probably be expanded, but I didn't want to bloat the CONTRIBUTING guide too much. Let me know what you think.

@emdupre @choldgraf Have you seen other repositories that are keeping a Python 2-compatible branch alive for a bit, and how they're directing people to this? A quick search through some major packages has brought up nothing.

[codecov]

Codecov Report

Merging #3115 into master will increase coverage by 0.63%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3115      +/-   ##
==========================================
+ Coverage   67.23%   67.86%   +0.63%     
==========================================
  Files         294      295       +1     
  Lines       39245    39263      +18     
  Branches     5162     5172      +10     
==========================================
+ Hits        26385    26646     +261     
+ Misses      12186    11912     -274     
- Partials      674      705      +31

Flag	Coverage Δ
#smoketests	51.23% <ø> (ø)	⬆️
#unittests	65.09% <ø> (+0.87%)	⬆️

Impacted Files	Coverage Δ
nipype/testing/__init__.py	88.88% <0%> (ø)
nipype/interfaces/spm/preprocess.py	56.93% <0%> (+0.32%)	⬆️
nipype/utils/filemanip.py	77.59% <0%> (+0.47%)	⬆️
nipype/interfaces/freesurfer/preprocess.py	66.04% <0%> (+0.98%)	⬆️
nipype/interfaces/freesurfer/model.py	64.19% <0%> (+1.12%)	⬆️
nipype/interfaces/spm/base.py	87% <0%> (+2%)	⬆️
nipype/interfaces/freesurfer/utils.py	62.88% <0%> (+2.13%)	⬆️
nipype/interfaces/io.py	59.67% <0%> (+6.11%)	⬆️
nipype/interfaces/dcm2nii.py	64.78% <0%> (+15.96%)	⬆️
nipype/interfaces/freesurfer/base.py	78.44% <0%> (+21.55%)	⬆️
... and 2 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 388f140...eb16dcc. Read the comment docs.

[choldgraf]

I don't know if any specifically. I know matplotlib backports a lot of stuff but nothing that's unique to python 2. I think at this point many packages are just telling users they'll drop support soon (or already have). Maintaining a separate python 2 branch seems like something you could do if there were dedicated funding for it, but to me feels too much work with volunteer labor

[effigies]

In my mind, we won't be backporting, just accepting PRs against the maint/1.3.x branch for people who need to fix bugs but can't use Py3.

If it becomes a serious burden, then we can consider a hard cutoff, but it seems unlikely given the overall activity level in nipype.

[effigies]

In the absence of outrage, I'll go ahead and merge this one.

[emdupre]

I also haven't seen any other libraries with this strategy, but it makes sense to me ! 👍