Carry the dtype of a Discrete space through a round trip

[DenisDrobyshev]

Description

_serialize_discrete writes "int64" for every Discrete space and
_deserialize_discrete ignores the field, so a space built with another dtype
comes back as a different space:

space = spaces.Discrete(5, dtype=np.int32)
reconstructed = deserialize_space(serialize_space(space))

reconstructed.dtype     # int64
reconstructed == space  # False

The comment on the line — # this seems to be hardcoded in Gymnasium — was
true when it was written. Discrete gained a dtype argument in Gymnasium
1.3.0, and DiscreteArray-style specs that default to int32 are common enough
that a dataset can carry one.

test_space_serialize_deserialize compares the two serialized strings rather
than the two spaces, so it stays green either way. That is why this went
unnoticed.

Changes

• Serialising writes str(space.dtype). No version guard is needed there:
  Discrete.dtype exists on every supported Gymnasium and is int64 on the
  older ones, which is exactly what was written before, so old and new files
  agree.

• Deserialising passes the dtype from Gymnasium 1.3.0 onwards. pyproject.toml
  declares gymnasium>=0.28.1, so on an older install a dataset that asks for
  a narrower dtype is reported:

  ValueError: The dataset declares a Discrete space of dtype int32, which needs
  Gymnasium 1.3.0 or later; 1.2.0 is installed.
  

  Silently widening it would hand back a space the dataset never described.
  Every file written before this change says int64, which matches the default,
  so nothing existing hits that path.

The version check is the same shape as the one in
Farama-Foundation/Shimmy#157: major and minor compared as integers rather than
as strings, so 1.10 stays above 1.3, and no dependency on packaging, which
is not in this project's requirements.

Tests

test_discrete_keeps_its_dtype, parametrised over int32 and int64. The int32
case fails on main and passes here; int64 passes either way, which is where
the two defaults coincide. The case is skipped with a reason below Gymnasium
1.3.0. tests/test_serialization.py is 21 passed, pre-commit run clean on
both files.

#345 touches the same two files, so whichever of the two lands second will need
a rebase — say the word and I will push it.