gh-38203: Change inheritance order in UniqueRepresentation

    
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Currently in `UniqueRepresentation`, the MRO is constructed by
```class UniqueRepresentation(CachedRepresentation,
WithEqualityById):```
However, in order to maximize the utility of the `WithEqualityById`
class, in particular with subclasses of `UniqueRepresentation`, we
should place it *first* in the MRO. This is useful in #37128, where this
resolves MRO resolution issues and not having to reimplement (or set
aliases) the behavior of `WithEqualityById`.

However, this change, while seemingly innocuous, does cause a problem
with one doctest dealing with very old pickles (Sage version <= 4.0) in
`combinat/root_system/cartan_type.py` concerning `class
CartanType_simple_finite:`. Currently, it is unclear why this change is
resulting in the error:
```
sage: from sage.misc.fpickle import unpickleModule
sage: pg_make_integer = unpickle_global('sage.rings.integer',
'make_integer')
sage: si2 = pg_make_integer('4')
sage: unpickle_build(si1,
{'tools':unpickleModule('sage.combinat.root_system.type_A'), 't':['A',
si2], 'letter':'A', 'n':si2})
Traceback (most recent call last):
  File "/sage/src/sage/doctest/forker.py", line 715, in _run
    self.compile_and_execute(example, compiler, test.globs)
  File "/sage/src/sage/doctest/forker.py", line 1147, in
compile_and_execute
    exec(compiled, globs)
  File "<doctest sage.combinat.root_system.cartan_type.CartanType_simple
_finite.__setstate__[5]>", line 1, in <module>
    unpickle_build(si1,
{'tools':unpickleModule('sage.combinat.root_system.type_A'), 't':['A',
si2], 'letter':'A', 'n':si2})
  File "/sage/src/sage/misc/explain_pickle.py", line 2442, in
unpickle_build
    setstate(state)
  File "/sage/src/sage/combinat/root_system/cartan_type.py", line 3106,
in __setstate__
    self.__class__ = T.__class__
TypeError: __class__ assignment: 'CartanType' object layout differs from
'CartanType_simple_finite'
```
However, since these are very old pickles and we cannot reproduce the
failure otherwise, we feel that it is prudent to drop support for this
and remove the corresponding class.

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### ⌛ Dependencies

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URL: #38203
Reported by: Enrique Manuel Artal Bartolo
Reviewer(s): Travis Scrimshaw