Regain Python 2.6 support

distro.py

@@ -518,7 +518,7 @@ def __init__(self, f):
         self._f = f
 
     def __get__(self, obj, owner):
-        assert obj is not None, 'call {} on an instance'.format(self._fname)
+        assert obj is not None, 'call {0} on an instance'.format(self._fname)
         ret = obj.__dict__[self._fname] = self._f(obj)
         return ret
 
@@ -917,6 +917,14 @@ def _parse_os_release_content(lines):
                 pass
         return props
 
+    def check_output(*args, **kwargs):
+        """Run command with arguments and return its output as a byte string.
+
+        Same as subprocess.check_output(), but works before Python 2.7."""
+
+        p = subprocess.Popen(stdout=subprocess.PIPE, *args, **kwargs)
+        return p.communicate()[0].strip()
+
     @cached_property
     def _lsb_release_info(self):
         """
@@ -930,7 +938,7 @@ def _lsb_release_info(self):
         with open(os.devnull, 'w') as devnull:
             try:
                 cmd = ('lsb_release', '-a')
-                stdout = subprocess.check_output(cmd, stderr=devnull)
+                stdout = check_output(cmd, stderr=devnull)
             except OSError:  # Command not found
                 return {}
         content = stdout.decode(sys.getfilesystemencoding()).splitlines()