tools/pygrub: Fix pygrub's --entry flag for python3

string.atoi() has been deprecated since Python 2.0, has a big scary warning
in the python2.7 docs and is absent from python3 altogether. int() does the
same thing and is compatible with both.

See https://docs.python.org/2/library/string.html#string.atoi:

Signed-off-by: Alejandro Vallejo <alejandro.vallejo@cloud.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>

tools/pygrub/src/pygrub

@@ -731,7 +731,7 @@ class Grub:
 def get_entry_idx(cf, entry):
     # first, see if the given entry is numeric
     try:
-        idx = string.atoi(entry)
+        idx = int(entry)
         return idx
     except ValueError:
         pass