setup.py: add helpful comment on line that is a SyntaxError in Python < 2.6.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>

setup.py

@@ -279,7 +279,7 @@ def run_command(args, cwd=None, verbose=False):
     try:
         # remember shell=False, so use git.cmd on windows, not just git
         p = subprocess.Popen(args, stdout=subprocess.PIPE, cwd=cwd)
-    except EnvironmentError as e:
+    except EnvironmentError as e:  # if this gives a SyntaxError, note that Tahoe-LAFS requires Python 2.6+
         if verbose:
             print("unable to run %s" % args[0])
             print(e)