sort.py: use script name in usage/main docstring

Note: This depends on `os.path` and `sys.argv`, so the imports have to
appear before the docstring.  In which case, the docstring has to be
explicitly assigned to `__doc__` (as it ceases to be the first statement
in the file).

Example outputs of the script name (using `print(argv[0]); return`:

    $ ./contrib/sort.py
    ./contrib/sort.py
    $ python contrib/sort.sh
    contrib/sort.py
    $ (cd contrib && ./sort.py)
    ./sort.py

contrib/sort.py

@@ -2,33 +2,35 @@
 # This file is part of Firejail project
 # Copyright (C) 2014-2022 Firejail Authors
 # License GPL v2
-"""\
+
+# Requirements:
+#  python >= 3.6
+from os import path
+from sys import argv, exit as sys_exit, stderr
+
+__doc__ = f"""\
 Sort the arguments of the commands that support multiple arguments in profiles;
 the following commands are supported:
 
     private-bin, private-etc, private-lib, caps.drop, caps.keep, seccomp.drop,
     seccomp.drop, protocol
 
-Usage: ./sort.py [/path/to/profile ...]
+Usage: {path.basename(argv[0])} [/path/to/profile ...]
 
 Keep in mind that this will overwrite your profile(s).
 
 Examples:
-    $ ./sort.py MyAwesomeProfile.profile
-    $ ./sort.py new_profile.profile second_new_profile.profile
-    $ ./sort.py ~/.config/firejail/*.{profile,inc,local}
-    $ sudo ./sort.py /etc/firejail/*.{profile,inc,local}
+    $ {argv[0]} MyAwesomeProfile.profile
+    $ {argv[0]} new_profile.profile second_new_profile.profile
+    $ {argv[0]} ~/.config/firejail/*.{{profile,inc,local}}
+    $ sudo {argv[0]} /etc/firejail/*.{{profile,inc,local}}
 
 Exit Codes:
   0: Success: No profiles needed fixing.
   1: Error: One or more profiles could not be processed correctly.
   101: Info: One or more profiles were fixed.
 """
 
-# Requirements:
-#  python >= 3.6
-from sys import argv, exit as sys_exit, stderr
-
 
 def sort_alphabetical(original_items):
     items = original_items.split(",")