fix: Python 3.8 support (#1918)

There were recently some type annotation changes that weren't compatible
with python 3.8. This PR fixes those, and adds back 3.8 tests using
github actions

---------

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Author: daniel-sanche

.github/workflows/unittest.yml

@@ -0,0 +1,76 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+on:
+  pull_request:
+    branches:
+      - main
+
+permissions:
+  contents: read
+  
+name: unittest
+jobs:
+  unit:
+    runs-on: ubuntu-22.04
+    strategy:
+      matrix:
+        python: ['3.8']
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+    - name: Setup Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: ${{ matrix.python }}
+    - name: Install nox
+      run: |
+        python -m pip install --upgrade setuptools pip wheel
+        python -m pip install nox
+    - name: Run unit tests
+      env:
+        COVERAGE_FILE: .coverage-${{ matrix.python }}
+      run: |
+        nox -s unit-${{ matrix.python }}
+    - name: Upload coverage results
+      uses: actions/upload-artifact@v4
+      with:
+        name: coverage-artifact-${{ matrix.python }}
+        path: .coverage-${{ matrix.python }}
+        include-hidden-files: true
+
+  cover:
+    runs-on: ubuntu-latest
+    needs:
+        - unit
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v4
+    - name: Setup Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3.13"
+    - name: Install coverage
+      run: |
+        python -m pip install --upgrade setuptools pip wheel
+        python -m pip install coverage
+    - name: Download coverage results
+      uses: actions/download-artifact@v4
+      with:
+        path: .coverage-results/
+    - name: Report coverage results
+      run: |
+        find .coverage-results -type f -name '*.zip' -exec unzip {} \;
+        coverage combine .coverage-results/**/.coverage*
+        coverage report --show-missing --fail-under=99

google/auth/_default.py

@@ -16,13 +16,13 @@
 
 Implements application default credentials and project ID detection.
 """
+from __future__ import annotations
 
-from collections.abc import Sequence
 import io
 import json
 import logging
 import os
-from typing import Optional, TYPE_CHECKING
+from typing import Optional, Sequence, TYPE_CHECKING
 import warnings
 
 from google.auth import environment_vars

google/oauth2/id_token.py

@@ -54,6 +54,7 @@
     http://openid.net/specs/openid-connect-core-1_0.html#IDToken
 .. _CacheControl: https://cachecontrol.readthedocs.io
 """
+from __future__ import annotations
 
 import http.client as http_client
 import json

noxfile.py

@@ -32,9 +32,16 @@
 ]
 
 DEFAULT_PYTHON_VERSION = "3.14"
-# TODO(https://github.com/googleapis/google-auth-library-python/issues/1787):
-# Remove or restore testing for Python 3.7/3.8
-UNIT_TEST_PYTHON_VERSIONS = ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+UNIT_TEST_PYTHON_VERSIONS = [
+    "3.7",
+    "3.8",
+    "3.9",
+    "3.10",
+    "3.11",
+    "3.12",
+    "3.13",
+    "3.14",
+]
 
 # Error if a python version is missing
 nox.options.error_on_missing_interpreters = True
@@ -44,8 +51,6 @@
     "lint",
     "blacken",
     "mypy",
-    # TODO(https://github.com/googleapis/google-auth-library-python/issues/1787):
-    # Remove or restore testing for Python 3.7/3.8
     "unit-3.9",
     "unit-3.10",
     "unit-3.11",