gh-36547: expurge parent_old from cryptosystem
    
This is removing the import of "parent_old" in crypto folder.

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36547
Reported by: Frédéric Chapoton
Reviewer(s): Kwankyu Lee

Author: Release Manager

src/sage/crypto/cryptosystem.py

@@ -5,7 +5,9 @@
 This module contains base classes for various cryptosystems, including
 symmetric key and public-key cryptosystems. The classes defined in this
 module should not be called directly. It is the responsibility of child
-classes to implement specific cryptosystems. Take for example the
+classes to implement specific cryptosystems.
+
+Take for example the
 Hill or matrix cryptosystem as implemented in
 :class:`HillCryptosystem <sage.crypto.classical.HillCryptosystem>`. It is a
 symmetric key cipher so
@@ -27,22 +29,21 @@ class of
     | + VigenereCryptosystem
     + PublicKeyCryptosystem
 """
-
-#*****************************************************************************
+# ****************************************************************************
 #       Copyright (C) 2007 David Kohel <[email protected]>
 #
 #  Distributed under the terms of the GNU General Public License (GPL)
 #
-#                  http://www.gnu.org/licenses/
-#*****************************************************************************
-
-import sage.structure.parent_old as parent_old
+#                  https://www.gnu.org/licenses/
+# ****************************************************************************
 from sage.sets.set import Set_generic
 
-class Cryptosystem(parent_old.Parent, Set_generic):
+
+class Cryptosystem(Set_generic):
     r"""
     A base cryptosystem class. This is meant to be extended by other
     specialized child classes that implement specific cryptosystems.
+
     A cryptosystem is a pair of maps
 
     .. MATH::
@@ -143,7 +144,9 @@ def __init__(self, plaintext_space, ciphertext_space, key_space,
 
     def __eq__(self, right):
         r"""
-        Comparing ``self`` with ``right``. Two ``Cryptosystem`` objects
+        Comparing ``self`` with ``right``.
+
+        Two ``Cryptosystem`` objects
         are the same if they satisfy all of these conditions:
 
         - share the same type
@@ -323,7 +326,9 @@ def key_space(self):
 
     def block_length(self):
         r"""
-        Return the block length of this cryptosystem. For some cryptosystems
+        Return the block length of this cryptosystem.
+
+        For some cryptosystems
         this is not relevant, in which case the block length defaults to 1.
 
         EXAMPLES:
@@ -348,6 +353,7 @@ def period(self):
             raise TypeError("Argument has no associated period.")
         return self._period
 
+
 class SymmetricKeyCryptosystem(Cryptosystem):
     r"""
     The base class for symmetric key, or secret key, cryptosystems.
@@ -373,6 +379,7 @@ def alphabet_size(self):
         """
         return self._cipher_domain.ngens()
 
+
 class PublicKeyCryptosystem(Cryptosystem):
     r"""
     The base class for asymmetric or public-key cryptosystems.