pip: "conflict with the backend dependencies" error running pip install in editable mode with pyproject.toml / PEP-517 enabled project

Environment

• pip version: 19.0.2
• Python version: 2.7.15
• OS: travis xenial

Description

Around a day ago, my cron-triggered builds on Travis for one projects started failing with Some build dependencies for file:///home/travis/build/cjw296/carly conflict with the backend dependencies: attrs==19.1.0 is incompatible with attrs<19.0 ,>=18.2. when being installed by poetry. Looks like under the hood, poetry is calling ['/home/travis/virtualenv/python2.7.15/bin/pip', 'install', '-e', '/ home/travis/build/cjw296/carly', '--no-deps'] which would suggest that dependencies should be ignored.

The full build log for this first failure is here: https://travis-ci.org/cjw296/carly/jobs/501041034

There is a poetry issue open for this here: https://github.com/sdispater/poetry/issues/826

It’s unclear whether this is a poetry bug or a pip bug, but it appears that either downgrading to pip 18.x or removing the build-system section from pyproject.toml stop this happening.

What’s super confusing is that my particular builds are run nightly and they only started failing on 3rd March, even though neither pip nor poetry had any releases around that date, and I haven’t made any code changes to carly since 20th Feb.

Expected behavior

This build log shows expected behaviour: https://travis-ci.org/cjw296/carly/jobs/500733549

How to Reproduce

These are the steps: https://github.com/cjw296/carly/blob/6ec7eedea9c8405ea6a61f733cb7ee9dfaa9b801/.travis.yml#L21-L23

These should be run against https://github.com/cjw296/carly/tree/6ec7eedea9c8405ea6a61f733cb7ee9dfaa9b801

Output

$ curl -sSL https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py | python
Retrieving Poetry metadata
# Welcome to Poetry!
This will download and install the latest version of Poetry,
a dependency and package manager for Python.
It will add the `poetry` command to Poetry's bin directory, located at:
$HOME/.poetry/bin
This path will then be added to your `PATH` environment variable by
modifying the profile files located at:
$HOME/.profile
$HOME/.bash_profile
You can uninstall at any time with `poetry self:uninstall`,
or by executing this script with the --uninstall option,
and these changes will be reverted.
Installing version: 0.12.11
  - Downloading poetry-0.12.11-linux.tar.gz (8.00MB)
Poetry (0.12.11) is installed now. Great!
To get started you need Poetry's bin directory ($HOME/.poetry/bin) in your `PATH`
environment variable. Next time you log in this will be done
automatically.
To configure your current shell run `source $HOME/.poetry/env`
install.2
0.00s$ source $HOME/.poetry/env
23.50s$ poetry install
Updating dependencies
Resolving dependencies... (5.7s)
Package operations: 12 installs, 1 update, 0 removals
Writing lock file
  - Installing idna (2.8)
  - Updating six (1.11.0 -> 1.12.0)
  - Installing automat (0.7.0)
  - Installing constantly (15.1.0)
  - Installing hyperlink (18.0.0)
  - Installing incremental (17.5.0)
  - Installing pyhamcrest (1.9.0)
  - Installing txaio (18.8.1)
  - Installing zope.interface (4.6.0)
  - Installing autobahn (18.12.1)
  - Installing coverage (4.4.2)
  - Installing testfixtures (6.6.0)
  - Installing twisted (18.9.0)
  - Installing carly (0.9.1)
                                                                                
[EnvCommandError]                                                
Command ['/home/travis/virtualenv/python2.7.15/bin/pip', 'install', '-e', '/        
home/travis/build/cjw296/carly', '--no-deps'] errored with the following out  
put:                                                                          
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020.  
 Please upgrade your Python as Python 2.7 won't be maintained after that dat  
e. A future version of pip will drop support for Python 2.7.                  
Obtaining file:///home/travis/build/cjw296/carly                              
  Installing build dependencies: started                                      
  Installing build dependencies: finished with status 'done'                  
  Getting requirements to build wheel: started                                
  Getting requirements to build wheel: finished with status 'done'            
Some build dependencies for file:///home/travis/build/cjw296/carly conflict   
with the backend dependencies: attrs==19.1.0 is incompatible with attrs<19.0  
,>=18.2.                                                                      
                                                                                
install [--no-dev] [--dry-run] [-E|--extras EXTRAS] [--develop DEVELOP]
The command "poetry install" failed and exited with 1 during .
Your build has been stopped.