Update tests for Python 3.9 on Ubuntu 24.04

Previously Python 3.9 was only supported on the Heroku-22
stack (Ubuntu 22.04) and older, since it reaches EOL in Oct 2025.

However, support for Python 3.9 on Heroku-24 (Ubuntu 24.04) was
just added, by the building/uploading of the binaries in:
heroku/heroku-buildpack-python#1656

As such, the tests need updating to reflect that CNB builds will no
longer fail with an unavailable Python error when using Python 3.9
on `heroku/builder:24`.

GUS-W-16917997.

tests/django_test.rs

@@ -26,7 +26,7 @@ fn django_staticfiles_latest_django() {
     );
 }
 
-// This tests the oldest Django version that works on Python 3.10 (which is the
+// This tests the oldest Django version that works on Python 3.9 (which is the
 // oldest Python that is available on all of our supported builders).
 #[test]
 #[ignore = "integration test"]
@@ -40,6 +40,7 @@ fn django_staticfiles_legacy_django() {
                 indoc! {"
                     [Generating Django static files]
                     Running 'manage.py collectstatic'
+                    Linking '/workspace/testapp/static/robots.txt'
                     
                     1 static file symlinked to '/workspace/staticfiles'.
                 "}

tests/fixtures/django_staticfiles_legacy_django/.python-version

@@ -1 +1 @@
-3.10
+3.9

tests/fixtures/django_staticfiles_legacy_django/requirements.txt

@@ -1,3 +1,3 @@
-# This is the oldest Django version that works on Python 3.10 (which is the
+# This is the oldest Django version that works on Python 3.9 (which is the
 # oldest Python that is available on all of our supported builders).
-Django==2.1.15
+Django==1.8.19

tests/fixtures/runtime_txt_and_python_version_file/runtime.txt

@@ -1 +1 @@
-python-3.10.0
+python-3.9.0

tests/python_version_test.rs

@@ -69,14 +69,7 @@ fn python_3_8() {
 #[test]
 #[ignore = "integration test"]
 fn python_3_9() {
-    // Python 3.9 is only available on Heroku-22 and older.
-    let fixture = "tests/fixtures/python_3.9";
-    match builder().as_str() {
-        "heroku/builder:20" | "heroku/builder:22" => {
-            builds_with_python_version(fixture, &LATEST_PYTHON_3_9);
-        }
-        _ => rejects_non_existent_python_version(fixture, &LATEST_PYTHON_3_9),
-    };
+    builds_with_python_version("tests/fixtures/python_3.9", &LATEST_PYTHON_3_9);
 }
 
 #[test]
@@ -309,10 +302,10 @@ fn runtime_txt() {
             context.pack_stdout,
             indoc! {"
                 [Determining Python version]
-                Using Python version 3.10.0 specified in runtime.txt
+                Using Python version 3.9.0 specified in runtime.txt
                 
                 [Installing Python]
-                Installing Python 3.10.0
+                Installing Python 3.9.0
             "}
         );
     });