Add 'fast' extra_require

Allows other projects to demand a Cython-compiled lxml without having to
add Cython to their own list of dependencies. By specifying
extra_require, other projects just have to add this to their setup.py:

    install_requires = ["lxml[fast]"],

They don't need to track the correct version anymore. And they don't
need to fix should lxml move to Shed_Skin or another Py-to-C compiler.

See http://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

1 files changed, 5 insertions, 0 deletions

diff --git a/setup.py b/setup.py
index 17c4c7be..6f6d162f 100644
--- a/setup.py
+++ b/setup.py
@@ -49,6 +49,11 @@ extra_options = {}
 if 'setuptools' in sys.modules:
     extra_options['zip_safe'] = False
 
+    import pkg_resources
+    extra_options['extra_require'] = {
+            'fast': map(str, pkg_resources.parse_requirements(open("requirements.txt", "r")))
+        }
+
 extra_options.update(setupinfo.extra_setup_args())
 
 extra_options['package_data'] = {