Add a tox config and some very primitive pip-grep and pip-diff tests

pip-grep and pip-diff were previously not tested at all. Ideally they
would be unit-tested from Python, however they'd need some refactoring
(they can't even be imported at present), so for now this is better than
nothing.

Another option for the future is possibly:
https://scripttest.readthedocs.io

.gitignore

@@ -0,0 +1,3 @@
+/.tox
+/*.egg-info
+*.pyc

README.rst

@@ -23,3 +23,11 @@ Possible Future Commands
 ------------------------
 
 - Install with blacklisting support (wsgiref, distribute, setuptools).
+
+Development
+-----------
+
+To run the tests:
+
+1. ``pip install -r requirements.txt``
+2. ``tox``

requirements.txt

@@ -1 +1,2 @@
 docopt==0.6.2
+tox==2.3.1

setup.py

@@ -19,6 +19,7 @@ setup(
     zip_safe=False,
     platforms='any',
     install_requires=['docopt', 'pip>=1.5.0'],
+    tests_require=['tox'],
     classifiers=[
         # As from https://pypi.python.org/pypi?%3Aaction=list_classifiers
         #'Development Status :: 1 - Planning',

tests/test-requirements.txt

@@ -0,0 +1,3 @@
+cffi
+django
+requests[security]

tests/test-requirements2.txt

@@ -0,0 +1,3 @@
+Django==1.9.6
+gunicorn
+requests[security]

tox.ini

@@ -0,0 +1,34 @@
+[tox]
+# To reduce the size of the test matrix, tests the following pip versions:
+#  * all of the latest major version releases (8.x)
+#  * only the first and last point release of each older major version
+#  * plus the latest distro version available via LTS Ubuntu package managers:
+#    (1.5.4 for Trusty, 8.1.1 for Xenial).
+envlist = pip{154,156,600,611,700,712,800,801,802,803,810,811,812}
+
+[testenv]
+# Speeds up pip install and reduces log spam, for compatible versions of pip.
+setenv = PIP_DISABLE_PIP_VERSION_CHECK=1
+
+deps =
+    pip154: pip==1.5.4
+    pip156: pip==1.5.6
+    pip600: pip==6.0.0
+    pip611: pip==6.1.1
+    pip700: pip==7.0.0
+    pip712: pip==7.1.2
+    pip800: pip==8.0.0
+    pip801: pip==8.0.1
+    pip802: pip==8.0.2
+    pip803: pip==8.0.3
+    pip810: pip==8.1.0
+    pip811: pip==8.1.1
+    pip812: pip==8.1.2
+
+# TODO: Replace with something like https://scripttest.readthedocs.io or else
+# rework the pip-grep and pip-diff scripts so they can more easily be unit-tested.
+commands =
+    pip-grep tests/test-requirements.txt cffi
+    pip-grep tests/test-requirements.txt requests
+    pip-diff --fresh tests/test-requirements.txt tests/test-requirements2.txt
+    pip-diff --stale tests/test-requirements.txt tests/test-requirements2.txt