Guard deprecated astype copy keyword by pandas version in encode_values_into_bin_idx

[xyf5432]

PR: Guard deprecated astype copy keyword by pandas version in encode_values_into_bin_idx

Fixes #47

Changes

Guard the deprecated copy keyword of Series.astype by pandas
version in DateTimeAttribute.encode_values_into_bin_idx, so behaviour
is preserved on every supported version:

• DataSynthesizer/datatypes/DateTimeAttribute.py: module-level
  PANDAS_VERSION = Version(pd.__version__) (from packaging, already
  a transitive dependency of pandas), and

  if PANDAS_VERSION < Version("3.0"):
      return encoded.astype(int, copy=False)
  else:
      return encoded.astype(int)

  — the same pattern as Fix pandas 3 string dtype compatibility and CI flakes NVIDIA/cuml#8142.

Verification

The copy keyword never affects the result of astype on any
pandas version, only whether an extra copy is made; the branch keeps
that behaviour exactly as it was on each side of the boundary. Runtime
checks on pandas 2.3.3 and 3.0.5 (Series → fillna → branch):

• pandas 2.3.3 takes the copy=False branch, results identical.
• pandas 3.0.5 takes the plain branch: no Pandas4Warning is emitted,
  results identical (the copy keyword is a documented no-op under
  Copy-on-Write).
• The modified file passes python -m py_compile.