Trac #23075: copy(CombinatorialFreeModule.Element) broken by #22632

The following problem showed up with 8.0.beta8: the copy of an
IndexedFreeModuleElement seems to be not functional:
{{{
#!sage
sage: F=CombinatorialFreeModule(ZZ,ZZ)
sage: a=F.an_element() ; a
3*B[-1] + B[0] + 3*B[1]
sage: copy(a)
<repr(<sage.combinat.free_module.CombinatorialFreeModule_with_category.e
lement_class at 0x7f5d735e7808>) failed: AttributeError: 'NoneType'
object has no attribute 'items'>
sage: type(copy(a)._monomial_coefficients)
<type 'NoneType'>
}}}

Part of Meta-ticket #13811: Support Python's `__copy__` / `__deepcopy__`
protocol

URL: https://trac.sagemath.org/23075
Reported by: cnassau
Ticket author(s): Travis Scrimshaw
Reviewer(s): Matthias Koeppe

src/sage/modules/with_basis/indexed_element.pyx

@@ -5,10 +5,11 @@ An element in an indexed free module
 AUTHORS:
 
 - Travis Scrimshaw (03-2017): Moved code from :mod:`sage.combinat.free_module`.
+- Travis Scrimshaw (29-08-2022): Implemented ``copy`` as the identity map.
 """
 
 #*****************************************************************************
-#       Copyright (C) 2017 Travis Scrimshaw <tcscrims at gmail.com>
+#       Copyright (C) 2017, 2022 Travis Scrimshaw <tcscrims at gmail.com>
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -180,6 +181,32 @@ cdef class IndexedFreeModuleElement(ModuleElement):
         for k, v in state[1].iteritems():
             setattr(self, k, v)
 
+    def __copy__(self):
+        r"""
+        Return ``self`` since ``self`` is immutable.
+
+        EXAMPLES::
+
+            sage: F = CombinatorialFreeModule(QQ, ['a','b','c'])
+            sage: x = F.an_element()
+            sage: copy(x) is x
+            True
+        """
+        return self
+
+    def __deepcopy__(self, memo=None):
+        r"""
+        Return ``self`` since ``self`` is immutable.
+
+        EXAMPLES::
+
+            sage: F = CombinatorialFreeModule(QQ, ['a','b','c'])
+            sage: x = F.an_element()
+            sage: deepcopy(x) is x
+            True
+        """
+        return self
+
     cpdef dict monomial_coefficients(self, bint copy=True):
         """
         Return the internal dictionary which has the combinatorial objects