Fix pip version to 2.7.

[marcelbuesing]

Platformio currently only supports python 2.7 not 3.0. This leads to issues when having both python2 and python3 installed.

I ran into the following error:

Installing platformio...
Collecting platformio
  Downloading platformio-3.5.0.tar.gz (91kB)
    100% |████████████████████████████████| 92kB 2.5MB/s 
    Complete output from command python setup.py egg_info:
    PlatformIO Core v3.5.0 does not run under Python version 3.6.4.
    Minimum supported version is 2.7, please upgrade Python.
    Python 3 is not yet supported.
    
    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-build-8vfq5sgq/platformio/

[Antti]

I don't think this will work for everyone.

[emosenkis]

Is there a way to make this work universally?

[skorokithakis]

Why wouldn't this work? In my experience, pip aliases itself to pip2.7 when installing, so this would only fail if you don't have pip installed. Am I wrong?

[skorokithakis]

Also, a way to avoid needing sudo for this is to attempt a user installation with --user. You'll need to refer to the full path as the directory might not be in the user's path afterwards, but that might be acceptable and it's cleaner than installing system-wide.

[BrendanBall]

I fixed this locally by creating a virtualenv, but this means I had to remove the --user from pip install platformio --user. imho if you're going to run this outside of docker you should use a virtualenv.