Don't install packages that could mess packaging up

CHANGELOG.md

@@ -1,5 +1,17 @@
 # Python Buildpack Changelog
 
+# 106
+
+Don't install packages that could mess up packaging.
+
+- The Python buildpack will automatically remove `six`, `pyparsing`, `appdirs`,
+  `setuptools`, and `distribute` from a `requirements.txt` file now, as these
+  packages are provided by the Python buildpack.
+
+# 105
+
+Improvements to output messaging.
+
 # 104
 
 General improvements.

bin/compile

@@ -153,6 +153,9 @@ if [ ! -f requirements.txt ] && [ ! -f Pipfile ]; then
   echo "-e ." > requirements.txt
 fi
 
+# Cleanup requirements.txt
+source $BIN_DIR/steps/setuptools
+
 # Fix egg-links.
 source $BIN_DIR/steps/eggpath-fix
 

bin/steps/setuptools

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+
+# Syntax sugar.
+source $BIN_DIR/utils
+
+pip-clean requirements.txt

vendor/pip-pop/pip-clean

@@ -0,0 +1,33 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+"""Usage:
+  pip-clean <req-file>
+
+Options:
+  -h --help     Show this screen.
+"""
+from docopt import docopt
+
+BAD_PACKAGES = ['appdirs', 'packaging', 'pyparsing', 'six', 'setuptools', 'distribute']
+
+
+def good_package(line):
+    package_name = line.split('=')[0].split('<')[0].split('>')[0].split(' ')[0].split('#')[0].split('\n')[0]
+    return package_name not in BAD_PACKAGES
+
+def main():
+    args = docopt(__doc__, version='pip-grep')
+    req_file = args['<req-file>']
+
+    with open(req_file, 'r') as f:
+        # Iterate over every line in the requirements file.
+        lines = [line for line in f if good_package(line)]
+
+    # Write the requirements file to disk.
+    with open(req_file, 'w') as f:
+        f.write(''.join(lines))
+
+
+if __name__ == '__main__':
+    main()