[file_utils] fix python shebang & run prospector

prospector is still complaining about bare excepts in unln.py.

file_utils/dirpacker.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # dirpacker.py, Copyright © 2014 Matteo Cypriani <mcy@lm7.fr>
 #
@@ -119,7 +119,7 @@ def create_bin(binnumber):
         files[sizes[size].pop()] = size
         # Delete the size from the dictionary if its last file name was just
         # popped:
-        if not len(sizes[size]):
+        if not sizes[size]:
             del sizes[size]
 
         binsize = newbinsize
@@ -194,7 +194,7 @@ if options.verbose:
 bins = []
 binnumber = options.first_bin_number
 binscreated = 0
-while len(sizes):
+while sizes:
     bins.append(create_bin(binnumber))
     binnumber += 1
     binscreated += 1

file_utils/unln.py

@@ -1,4 +1,4 @@
-#!/usr/bin/python3
+#!/usr/bin/env python3
 #
 # unln.py, Copyright © 2013 Matteo Cypriani <mcy@lm7.fr>
 #
@@ -177,6 +177,6 @@ for filename in options.filenames:
 
 
 # Display the list of files in error
-if len(error_filenames):
+if error_filenames:
     warn("The following regular files still have more than one hard link:")
     print(*error_filenames, sep="\n", file=sys.stderr)