setup.py: Add minimum pip requirement check

Older setuptools/pip fails on installation
due to missing syntax in them. This fixes
that problem by avoiding such installation.

Related to: coala#6017

.misc/appveyor.yml

@@ -42,7 +42,7 @@ install:
   # Check that we have the expected version and architecture for Python
   - "python --version"
   - "python -c \"import struct; print(struct.calcsize('P') * 8)\""
-  - "%CMD_IN_ENV% python -m pip install --upgrade setuptools==21 pip==9"
+  - "%CMD_IN_ENV% python -m pip install --upgrade setuptools==21 pip==9.0.1"
   - "%CMD_IN_ENV% python -m pip install -r test-requirements.txt \
     -r requirements.txt -r docs-requirements.txt"
 

.travis.yml

@@ -89,7 +89,7 @@ before_install:
   # Apart from builds with python 2.7 and 3.5, it installs setuptools
   # twice. So, it is required to uninstall one version manually.
   - pip uninstall setuptools --yes
-  - pip install pip==9 setuptools==21
+  - pip install pip==9.0.1 setuptools==21
   - python .misc/check_setuptools.py
 
   # https://github.com/coala/coala/issues/3183

setup.py

@@ -8,6 +8,7 @@
 from os import getenv
 from subprocess import call
 
+import pip
 import setuptools.command.build_py
 from setuptools import find_packages, setup
 from setuptools.command.test import test as TestCommand
@@ -437,6 +438,10 @@ def read_requirements(filename):
 })
 
 if __name__ == '__main__':
+    pip_version = LooseVersion(pip.__version__)
+    if pip_version < LooseVersion('9.0.1'):
+        raise RuntimeError('Version of pip is less than 9.0.1. '
+                           'Consider upgrading pip to pip~=9.0.1')
     setup(name='coala',
           version=VERSION,
           description=DESCRIPTION,