ci: pin Python 3.9 leg to ubuntu-24.04

ubuntu-latest now resolves to Ubuntu 26.04, which has no CPython 3.9
build in actions/python-versions, so setup-python failed with "version
'3.9' ... not found for Ubuntu 26.04". The package still declares
python_requires=">=3.9", so keep the 3.9 leg but run it on ubuntu-24.04
(the newest image that ships 3.9); 3.10-3.13 stay on ubuntu-latest.

Author: zhansheng.lzs

.github/workflows/unit_test.yml

@@ -15,8 +15,19 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
-        python-version: [ '3.9', '3.10', '3.11', '3.12', '3.13' ]
+        # ubuntu-latest (26.04) ships no CPython 3.9 build, so pin the
+        # still-supported 3.9 leg to 24.04 and keep the rest on latest.
+        include:
+          - os: ubuntu-24.04
+            python-version: '3.9'
+          - os: ubuntu-latest
+            python-version: '3.10'
+          - os: ubuntu-latest
+            python-version: '3.11'
+          - os: ubuntu-latest
+            python-version: '3.12'
+          - os: ubuntu-latest
+            python-version: '3.13'
 
     steps:
       - name: Checkout repository
@@ -68,8 +79,19 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
-        python-version: [ '3.9', '3.10', '3.11', '3.12', '3.13' ]
+        # ubuntu-latest (26.04) ships no CPython 3.9 build, so pin the
+        # still-supported 3.9 leg to 24.04 and keep the rest on latest.
+        include:
+          - os: ubuntu-24.04
+            python-version: '3.9'
+          - os: ubuntu-latest
+            python-version: '3.10'
+          - os: ubuntu-latest
+            python-version: '3.11'
+          - os: ubuntu-latest
+            python-version: '3.12'
+          - os: ubuntu-latest
+            python-version: '3.13'
 
     steps:
       - name: Checkout repository