Use a requirements file to make dev easy

Setting up packages needed for hacking can now just be ~one step

README.rst

@@ -43,7 +43,13 @@ Development
 
 To install the latest development version::
 
-  pip install setuptools cffi tox 'cython>=0.23.4' git+git://github.com/gevent/gevent.git#egg=gevent
+  pip install setuptools 'cython>=0.23.4' git+git://github.com/gevent/gevent.git#egg=gevent
+
+To hack on gevent::
+
+  $ git clone https://github.com/gevent/gevent.git
+  $ cd gevent
+  $ pip install -r dev-requirements.txt
 
 .. note::
    You must have Cython, a C compiler, and the Python

dev-requirements.txt

+cython>=0.23.4
+greenlet>=0.4.9
+coverage>=4.0
+cffi
+setuptools
+-e .