gnu: Add python-slugify.

* gnu/packages/python-web.scm (python-slugify): New variable.
* gnu/packages/patches/python-slugify-depend-on-unidecode.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/packages/patches/python-slugify-depend-on-unidecode.patch

@@ -0,0 +1,22 @@
+diff --git a/setup.py b/setup.py
+index 4800173..6bdd77f 100755
+--- a/setup.py
++++ b/setup.py
+@@ -14,8 +14,7 @@ url = 'https://github.com/un33k/python-slugify'
+ author = 'Val Neekman'
+ author_email = 'info@neekware.com'
+ license = 'MIT'
+-install_requires = ['text-unidecode==1.2']
+-extras_require = {'unidecode': ['Unidecode==1.0.23']}
++install_requires = ['Unidecode']
+ 
+ classifiers = [
+     'Development Status :: 5 - Production/Stable',
+@@ -67,7 +66,6 @@ setup(
+     author_email=author_email,
+     packages=find_packages(exclude=EXCLUDE_FROM_PACKAGES),
+     install_requires=install_requires,
+-    extras_require=extras_require,
+     classifiers=classifiers,
+     entry_points={'console_scripts': ['slugify=slugify.slugify:main']},
+ )