ImportError: No module named build_clib / pip installs setuptools 36.0.1 when directed to install 30.0.0

* Pip version: 9.0.1

* Python version: 2.7.13

* Operating system: Ubuntu 14.04 (on Travis CI)



### Description (including what I've run):



We test our project on Travis with the minimum Python package versions we specify. A few days ago, it started consistently failing the `pip install .` for our project, in the `setup.py egg_info` step. It turns out that running `python setup.py egg_info`  reproduces the same error, as shown here:



```

python setup.py egg_info

ERROR:root:Error parsing

Traceback (most recent call last):

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/pbr/core.py", line 111, in pbr

    attrs = util.cfg_to_args(path, dist.script_args)

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/pbr/util.py", line 264, in cfg_to_args

    wrap_commands(kwargs)

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/pbr/util.py", line 566, in wrap_commands

    cmdclass = ep.resolve()

  File "/home/travis/virtualenv/python2.7.13/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2293, in resolve

    module = __import__(self.module_name, fromlist=['__name__'], level=0)

ImportError: No module named build_clib

error in setup command: Error parsing /home/travis/build/zhmcclient/python-zhmcclient/setup.cfg: ImportError: No module named build_clib

```



This happens on Travis with Ubuntu 14.04 and Python 2.7.13 (it uses a virtuel env).



On a locally set up Ubuntu 14.04 with a manually installed Python 2.7.13, in a virtual env, and with the same Python packages and versions installed in that virtual env, and the same pip version, I was not able to reproduce it so far.



The set of Python packages in the virtual env, when it happens on Travis, is:



```

funcsigs (1.0.2)

mock (2.0.0)

nose (1.3.7)

numpy (1.13.1)

pbr (1.10.0)

pip (9.0.1)

py (1.4.34)

pytest (3.2.1)

setuptools (36.0.1)

six (1.10.0)

wheel (0.29.0)

```



With this package state, we then reinstall pip, setuptools, wheel and pbr in single `pip install` commands, to the desired minimum versions. This is done separately from the project install, in order to make sure the install process itself also uses the minimum versions:



```

pip install --upgrade pip

pip install -c minimum-constraints.txt pip

pip install -c minimum-constraints.txt setuptools

pip install -c minimum-constraints.txt wheel

pip install -c minimum-constraints.txt pbr

```



We then install our own project:



```

pip install -c minimum-constraints.txt .

```



Looking at the log reveals that the `pip install setuptools` installs 36.0.1 when asked to install 30.0.0 (via the constraints file):



```

pip install -c minimum-constraints.txt setuptools

Collecting setuptools===30.0.0 (from -c minimum-constraints.txt (line 21))

  Downloading setuptools-30.0.0-py2.py3-none-any.whl (472kB)

    100% |████████████████████████████████| 481kB 2.4MB/s 

Installing collected packages: setuptools

  Found existing install

[]