Vendor deprecate_kwarg to fix pandas 3.0 ImportError (#1005)

[Jaypatel1511]

Summary

Fixes #1005 — pandas-datareader imported deprecate_kwarg from pandas.util._decorators, a private internal API that is being removed in pandas 3.0. This causes an ImportError on pandas >= 3.0, breaking every import of the library.

Root cause

# pandas_datareader/data.py
from pandas.util._decorators import deprecate_kwarg  # private API, removed in pandas 3.0

Fix

Vendor the deprecate_kwarg implementation directly into pandas_datareader.compat (matching the behaviour of the pandas original), and update data.py to import from there:

# pandas_datareader/data.py
from pandas_datareader.compat import deprecate_kwarg  # no private pandas dependency

The vendored implementation in compat/__init__.py is a faithful port of the pandas original, supporting:

• Simple old → new kwarg rename with FutureWarning
• mapping dict or callable for value translation
• TypeError when both old and new kwargs are passed simultaneously
• new_arg_name=None for pure deprecation warnings

Test plan

• 8 new unit tests in tests/test_deprecate_kwarg.py covering all decorator behaviours
• One AST-based test asserting data.py no longer references pandas.util._decorators
• All 8 tests pass on Python 3.9 with pandas 2.x