setup: Add an [all_importers] extra for ease of installation.

setup.py

@@ -1,6 +1,5 @@
 #!/usr/bin/env python3
 
-import itertools
 import sys
 
 from setuptools import setup, find_packages
@@ -17,11 +16,15 @@ REQUIREMENTS = {
     },
 }
 
+all_extras_require = [
+    req for reqlist in REQUIREMENTS['extras_require'].values() for req in reqlist
+]
 
+REQUIREMENTS['extras_require']['all_importers'] = all_extras_require
 REQUIREMENTS['tests_require'] = [
     'pytest',
     'PyYAML',
-    *itertools.chain(*REQUIREMENTS['extras_require'].values()),
+    *all_extras_require,
 ]
 
 setup(