Apply follow-up changes for Python 3.8 removal

This commit addresses items missed in the initial Python 3.8 removal:

* Adds `kokoro/presubmit/system-3.9.cfg`.
* Updates example commands in `CONTRIBUTING.rst`.
* Modifies the warning in `pandas_gbq/__init__.py` for Python < 3.9.
* Updates Python versions in `owlbot.py`.
* Removes 3.8-specific line from `samples/snippets/requirements.txt`.
* Populates `testing/constraints-3.9.txt` with correct lower bounds.

Author: google-labs-jules[bot]

.kokoro/presubmit/system-3.9.cfg

@@ -0,0 +1,7 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Only run the following session(s)
+env_vars: {
+  key: "NOX_SESSION"
+  value: "system-3.9"
+}

CONTRIBUTING.rst

@@ -195,11 +195,11 @@ configure them just like the System Tests.
 
    # Run all tests in a folder
    $ cd samples/snippets
-   $ nox -s py-3.8
+   $ nox -s py-3.9
 
    # Run a single sample test
    $ cd samples/snippets
-   $ nox -s py-3.8 -- -k <name of test>
+   $ nox -s py-3.9 -- -k <name of test>
 
 ********************************************
 Note About ``README`` as it pertains to PyPI

owlbot.py

@@ -35,8 +35,8 @@
 extras = ["tqdm", "geopandas"]
 templated_files = common.py_library(
     default_python_version="3.10",
-    unit_test_python_versions=["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"],
-    system_test_python_versions=["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"],
+    unit_test_python_versions=["3.9", "3.10", "3.11", "3.12", "3.13"],
+    system_test_python_versions=["3.9", "3.10", "3.11", "3.12", "3.13"],
     cov_level=96,
     unit_test_external_dependencies=["freezegun"],
     unit_test_extras=extras,

pandas_gbq/__init__.py

@@ -11,14 +11,14 @@
 from .gbq import read_gbq, to_gbq  # noqa
 
 sys_major, sys_minor, sys_micro = _versions_helpers.extract_runtime_version()
-if sys_major == 3 and sys_minor in (7, 8):
+if sys_major == 3 and sys_minor < 9:
     warnings.warn(
-        "The python-bigquery library will stop supporting Python 3.7 "
-        "and Python 3.8 in a future major release expected in Q4 2024. "
-        f"Your Python version is {sys_major}.{sys_minor}.{sys_micro}. We "
-        "recommend that you update soon to ensure ongoing support. For "
-        "more details, see: [Google Cloud Client Libraries Supported Python Versions policy](https://cloud.google.com/python/docs/supported-python-versions)",
-        PendingDeprecationWarning,
+        "pandas-gbq will stop supporting Python versions older than 3.9 "
+        "in a future release. Your Python version is "
+        f"{sys_major}.{sys_minor}.{sys_micro}. Please update "
+        "to Python 3.9 or newer to ensure ongoing support. For more details, "
+        "see: https://cloud.google.com/python/docs/supported-python-versions",
+        FutureWarning,
     )
 
 __version__ = pandas_gbq_version.__version__

samples/snippets/requirements.txt

@@ -1,6 +1,5 @@
 google-cloud-bigquery-storage==2.29.1
 google-cloud-bigquery==3.30.0
 pandas-gbq==0.28.0
-pandas===2.0.3; python_version == '3.8'
 pandas==2.2.3; python_version >= '3.9'
 pyarrow==19.0.1; python_version >= '3.9'

testing/constraints-3.9.txt

@@ -1,2 +1,22 @@
-numpy==1.19.4
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+db-dtypes==1.0.4
+numpy==1.18.1
 pandas==1.1.4
+pyarrow==4.0.0
+pydata-google-auth==1.5.0
+google-api-core==2.10.2
+google-auth==2.13.0
+google-auth-oauthlib==0.7.0
+google-cloud-bigquery==3.4.2
+packaging==22.0.0
+# Extras
+google-cloud-bigquery-storage==2.16.2
+tqdm==4.23.0
+geopandas==0.9.0
+Shapely==1.8.4