ci: test with Python 3.15

Also moved python version to environment variable
so status checks will be named
"JSON-RPC Python tests (ubuntu-latest, latest)" instead of
"JSON-RPC Python tests (ubuntu-latest, 3.14)" and we don't have to
update the list of required status checks each time the version changes.
We already do the same for Rust.

.github/workflows/ci.yml

@@ -25,6 +25,8 @@ env:
   # Minimum Supported Rust Version
   MSRV: 1.89.0
 
+  PYTHON_VERSION: 3.15
+
 jobs:
   lint_rust:
     name: Lint Rust
@@ -268,11 +270,11 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Currently used Rust version.
+          # Currently used Python version.
           - os: ubuntu-latest
-            python: 3.14
+            python: latest
           - os: macos-latest
-            python: 3.14
+            python: latest
 
           # PyPy tests
           - os: ubuntu-latest
@@ -303,7 +305,7 @@ jobs:
       - name: Install python
         uses: actions/setup-python@v7.0.0
         with:
-          python-version: ${{ matrix.python }}
+          python-version: ${{ matrix.python == 'latest' && env.PYTHON_VERSION || matrix.python }}
 
       - name: Install tox
         run: pip install tox
@@ -324,11 +326,11 @@ jobs:
       matrix:
         include:
           - os: ubuntu-latest
-            python: 3.14
+            python: latest
           - os: macos-latest
-            python: 3.14
+            python: latest
           - os: windows-latest
-            python: 3.14
+            python: latest
 
           # PyPy tests
           - os: ubuntu-latest
@@ -351,7 +353,7 @@ jobs:
       - name: Install python
         uses: actions/setup-python@v7.0.0
         with:
-          python-version: ${{ matrix.python }}
+          python-version: ${{ matrix.python == 'latest' && env.PYTHON_VERSION || matrix.python }}
 
       - name: Install tox
         run: pip install tox