fix(google-auth-oauthlib): Drop support for Python 3.9 (googleapis#16939)

This PR updates `google-auth-oauthlib` to establish Python 3.10 as the
minimum supported version, dropping support for Python 3.9 and below.
### Changes
* Configuration: Updated `setup.py` and `noxfile.py` to require Python
3.10+ and remove references to Python 3.9.
* Documentation: Updated `README.rst` and `CONTRIBUTING.rst` to reflect
the new supported and unsupported Python versions.
* Constraints: Updated `constraints-3.10.txt` with minimum versions from
`constraints-3.9.txt` and deleted `constraints-3.9.txt`.

Fixes internal issue: http://b/482126936 🦕

Author: chalmerlowe

packages/google-auth-oauthlib/CONTRIBUTING.rst

@@ -22,7 +22,7 @@ In order to add a feature:
   documentation.
 
 - The feature must work fully on the following CPython versions:
-  3.9, 3.10, 3.11, 3.12, 3.13 and 3.14 on both UNIX and Windows.
+  3.10, 3.11, 3.12, 3.13 and 3.14 on both UNIX and Windows.
 
 - The feature must not add unnecessary dependencies (where
   "unnecessary" is of course subjective, but new dependencies should
@@ -197,14 +197,12 @@ Supported Python Versions
 
 We support:
 
--  `Python 3.9`_
 -  `Python 3.10`_
 -  `Python 3.11`_
 -  `Python 3.12`_
 -  `Python 3.13`_
 -  `Python 3.14`_
 
-.. _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/
@@ -217,7 +215,7 @@ Supported versions can be found in our ``noxfile.py`` `config`_.
 .. _config: https://github.com/googleapis/google-cloud-python/blob/main/noxfile.py
 
 
-We also explicitly decided to support Python 3 beginning with version 3.9.
+We also explicitly decided to support Python 3 beginning with version 3.10.
 Reasons for this include:
 
 -  Encouraging use of newest versions of Python 3

packages/google-auth-oauthlib/README.rst

@@ -29,13 +29,13 @@ The latest documentation is available at `google-auth-oauthlib.googleapis.dev`_.
 
 Supported Python Versions
 -------------------------
-Python >= 3.9
+Python >= 3.10
 
 
 Unsupported Python Versions
 ---------------------------
 
-Python == 2.7, Python <= 3.8.
+Python == 2.7, Python <= 3.9.
 
 
 License

packages/google-auth-oauthlib/noxfile.py

@@ -36,7 +36,6 @@
 DEFAULT_PYTHON_VERSION = "3.14"
 
 UNIT_TEST_PYTHON_VERSIONS: List[str] = [
-    "3.9",
     "3.10",
     "3.11",
     "3.12",

packages/google-auth-oauthlib/setup.py

@@ -54,12 +54,11 @@
             "google-oauthlib-tool" "=google_auth_oauthlib.tool.__main__:main [tool]"
         ]
     },
-    python_requires=">=3.9",
+    python_requires=">=3.10",
     license="Apache 2.0",
     keywords="google auth oauth client oauthlib",
     classifiers=[
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.9",
         "Programming Language :: Python :: 3.10",
         "Programming Language :: Python :: 3.11",
         "Programming Language :: Python :: 3.12",

packages/google-auth-oauthlib/testing/constraints-3.10.txt

@@ -0,0 +1,10 @@
+# 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
+google-auth==2.15.0
+requests-oauthlib==0.7.0
+click==6.0.0