fix: explicitly delare Python 3.13 support

Author: parthea

.github/workflows/unittest.yml

@@ -5,10 +5,13 @@ on:
 name: unittest
 jobs:
   unit:
-    runs-on: ubuntu-20.04
+    # TODO(https://github.com/googleapis/gapic-generator-python/issues/2303): use `ubuntu-latest` once this bug is fixed.
+    # Use ubuntu-22.04 until Python 3.7 is removed from the test matrix
+    # https://docs.github.com/en/actions/using-github-hosted-runners/using-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
-        python: ['3.6', '3.7', '3.8', '3.9', '3.10', '3.11', '3.12']
+        python: ['3.7', '3.8', '3.9', '3.10', '3.11', '3.12', '3.13']
     steps:
     - name: Checkout
       uses: actions/checkout@v4

.kokoro/samples/python3.6/common.cfg

@@ -22,7 +22,7 @@ In order to add a feature:
   documentation.
 
 - The feature must work fully on the following CPython versions:
-  3.6, 3.7, 3.8, 3.9, 3.10, 3.11 and 3.12 on both UNIX and Windows.
+  3.7, 3.8, 3.9, 3.10, 3.11, 3.12 and 3.13 on both UNIX and Windows.
 
 - The feature must not add unnecessary dependencies (where
   "unnecessary" is of course subjective, but new dependencies should
@@ -221,29 +221,29 @@ Supported Python Versions
 
 We support:
 
--  `Python 3.6`_
 -  `Python 3.7`_
 -  `Python 3.8`_
 -  `Python 3.9`_
 -  `Python 3.10`_
 -  `Python 3.11`_
 -  `Python 3.12`_
+-  `Python 3.13`_
 
-.. _Python 3.6: https://docs.python.org/3.6/
 .. _Python 3.7: https://docs.python.org/3.7/
 .. _Python 3.8: https://docs.python.org/3.8/
 .. _Python 3.9: https://docs.python.org/3.9/
 .. _Python 3.10: https://docs.python.org/3.10/
 .. _Python 3.11: https://docs.python.org/3.11/
 .. _Python 3.12: https://docs.python.org/3.12/
+.. _Python 3.13: https://docs.python.org/3.13/
 
 
 Supported versions can be found in our ``noxfile.py`` `config`_.
 
 .. _config: https://github.com/googleapis/google-auth-library-python-oauthlib/blob/main/noxfile.py
 
 
-We also explicitly decided to support Python 3 beginning with version 3.6.
+We also explicitly decided to support Python 3 beginning with version 3.7.
 Reasons for this include:
 
 -  Encouraging use of newest versions of Python 3

.kokoro/samples/python3.6/continuous.cfg

@@ -31,17 +31,20 @@ The latest documentation is available at `google-auth-oauthlib.googleapis.dev`_.
 
 Supported Python Versions
 -------------------------
-Python >= 3.6
+Python >= 3.7
 
 
 Unsupported Python Versions
 ---------------------------
 
-Python == 2.7, Python == 3.5.
+Python == 2.7, Python < 3.7.
 
 The last version of this library compatible with Python 2.7 and 3.5 is
 `google-auth-oauthlib==0.4.1`.
 
+The last version of this library compatible with Python 3.6 is
+`google-auth-oauthlib==1.2.2`.
+
 License
 -------
 

.kokoro/samples/python3.6/periodic-head.cfg

@@ -35,13 +35,13 @@
 DEFAULT_PYTHON_VERSION = "3.8"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
-    "3.6",
     "3.7",
     "3.8",
     "3.9",
     "3.10",
     "3.11",
     "3.12",
+    "3.13",
 ]
 UNIT_TEST_STANDARD_DEPENDENCIES = [
     "mock",
@@ -377,7 +377,7 @@ def docfx(session):
     )
 
 
-@nox.session(python="3.12")
+@nox.session(python=UNIT_TEST_PYTHON_VERSIONS[-1])
 @nox.parametrize(
     "protobuf_implementation",
     ["python", "upb", "cpp"],

.kokoro/samples/python3.6/periodic.cfg

@@ -10,12 +10,11 @@
     microgenerator=True,
     cov_level=99,
     unit_test_external_dependencies=["click"],
-    unit_test_python_versions=["3.6", "3.7", "3.8", "3.9", "3.10", "3.11", "3.12"],
+    unit_test_python_versions=["3.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"],
 )
 s.move(templated_files, excludes=[
     "docs/multiprocessing.rst",
     "README.rst",
-    ".github/workflows/unittest.yml" #remove this exclusion when removing 3.6 from unit test
 ])
 
 # Change black paths