chore: fix pytest crash on pandas 3.0 due to missing SettingWithCopyWarning (#2314)

Moved `ignore::pandas.errors.SettingWithCopyWarning` from `pytest.ini`
to `conftest.py` with a check for its existence. This fixes
`AttributeError: module 'pandas.errors' has no attribute
'SettingWithCopyWarning'` when running tests against pandas 3.0.

---
*PR created automatically by Jules for task
[1336882389279070030](https://jules.google.com/task/1336882389279070030)
started by @tswast*

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

conftest.py

@@ -14,13 +14,20 @@
 
 from __future__ import annotations
 
+import warnings
+
 import numpy as np
 import pandas as pd
 import pyarrow as pa
 import pytest
 
 import bigframes._config
 
+# Make sure SettingWithCopyWarning is ignored if it exists.
+# It was removed in pandas 3.0.
+if hasattr(pd.errors, "SettingWithCopyWarning"):
+    warnings.simplefilter("ignore", pd.errors.SettingWithCopyWarning)
+
 
 @pytest.fixture(scope="session")
 def polars_session_or_bpd():

pytest.ini

@@ -1,5 +1,3 @@
 [pytest]
 doctest_optionflags = NORMALIZE_WHITESPACE
-filterwarnings =
-    ignore::pandas.errors.SettingWithCopyWarning
 addopts = "--import-mode=importlib"