enforce Python 2.7 during installation

[tpltnt]

Hi there,

this patch raises a RuntimeError if Python 2.7 is not used for the installation. Multiple people recently tried to install tahoe-lafs on Python 3 (despite the hint in the readme) so I made it hard to do the wrong thing.

Cheers,
tpltnt

[meejah]

Thanks for the PR!
I don't think we actually depend on "2.7" (but definitely "less than 3.0" so maybe that would be a better check?)

[tpltnt]

I reduced the check to only test for python 2.

[codecov-io]

Codecov Report

Merging #414 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
- Coverage   88.01%   88.01%   -0.01%     
==========================================
  Files         146      146              
  Lines       27578    27578              
  Branches     3921     3921              
==========================================
- Hits        24274    24273       -1     
- Misses       2599     2600       +1     
  Partials      705      705

Impacted Files	Coverage Δ
src/allmydata/web/status.py	83.05% <0%> (-0.12%)	⬇️
src/allmydata/mutable/servermap.py	93.71% <0%> (ø)	⬆️

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[meejah]

Okay, I've confirmed that this gives a way nicer error-message if you're on Debian stable and just created a virtualenv with virtualenv --python=python3 foo.

Namely, it says "Python version 2 required" instead of some parse-error from the setup.py of zfec (because it happens to print something but using the python2 syntax).

[warner]

BTW, I opened https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2876, as this change breaks our Ubuntu/Xenial (16.04-LTS) buildbot, where tox is a python3 binary, and appears to unconditionally use it's native sys.executable (so py3) for the setup.py sdist command.

Our Travis config runs everything in a py2 virtualenv, and updates tox, so it didn't expose this problem. Our buildbot doesn't do pull requests, so we don't have a great way to discover problems like this until after the land. I guess we should make it a point to watch the buildbot after landing cross-platform-sensitive changes and then get comfortable with reverting a patch if it causes problems that aren't caught by travis or appveyor.