feat: Add support for Python 3.14

[parthea]

Fixes #14686
Closes #14806

Towards googleapis/librarian#2456

BEGIN_COMMIT_OVERRIDE
feat: Add support for Python 3.14
fix: Deprecate credentials_file argument
fix(deps): Require grpcio >= 1.33.2
fix(deps): Require grpcio >= 1.75.1 for Python 3.14
END_COMMIT_OVERRIDE

[parthea]

2fbb1a3 which restored samples/__init__.py fixed the following error

==================================== ERRORS ====================================
_______ ERROR collecting samples/pyarrow/append_rows_with_arrow_test.py ________
samples/pyarrow/append_rows_with_arrow_test.py:15: in 
    from . import append_rows_with_arrow
samples/pyarrow/append_rows_with_arrow.py:19: in 
    from google.cloud import bigquery
.nox/system-3-12/lib/python3.12/site-packages/google/cloud/bigquery/__init__.py:36: in 
    from google.cloud.bigquery.client import Client
.nox/system-3-12/lib/python3.12/site-packages/google/cloud/bigquery/client.py:65: in 
    from google.cloud.bigquery_storage_v1.services.big_query_read.client import (
google/cloud/bigquery_storage_v1/__init__.py:20: in 
    from google.cloud.bigquery_storage_v1 import client, types
google/cloud/bigquery_storage_v1/client.py:28: in 
    from google.cloud.bigquery_storage_v1 import reader
google/cloud/bigquery_storage_v1/reader.py:30: in 
    import pandas
.nox/system-3-12/lib/python3.12/site-packages/pandas/__init__.py:26: in 
    from pandas.compat import (
.nox/system-3-12/lib/python3.12/site-packages/pandas/compat/__init__.py:29: in 
    from pandas.compat.pyarrow import (
.nox/system-3-12/lib/python3.12/site-packages/pandas/compat/pyarrow.py:10: in 
    _palv = Version(Version(pa.__version__).base_version)
                            ^^^^^^^^^^^^^^
E   AttributeError: module 'pyarrow' has no attribute '__version__'

[tswast]

Aside: Will we update the templates to raise a warning when credentials_file is supplied? There's like 3 to pick from, but FutureWarning is the only one that's not ignored by default IIRC.

[parthea]

The warning was added as part of google-api-core in https://github.com/googleapis/python-api-core/pull/841/files and is already being emitted for users that have updated google-api-core. I decided not to include another warning in templates in googleapis/gapic-generator-python#2454 as it could be considered a duplicate. I chose DeprecationWarning as it seemed like the best fit at the time, but you're right, FutureWarning seems to be the one that we should use for end users based on the description in https://docs.python.org/3/library/warnings.html#warning-categories. Since google-api-core was already released, it could be disruptive to change the warning type from DeprecationWarning to FutureWarning now but I'll keep this in mind for future work.