GitHub Actions: Test on Python 3.14 release candidate 2

Python v3.14 -- October 7th
* https://www.python.org/download/pre-releases
* https://www.python.org/downloads/release/python-3140rc2

Author: cclauss

.github/workflows/main.yml

@@ -19,13 +19,15 @@ jobs:
         - '3.11'
         - '3.12'
         - '3.13'
+        - '3.14'
 
     steps:
     - uses: actions/checkout@v5
 
     - uses: actions/setup-python@v6
       with:
         python-version: ${{ matrix.python-version }}
+        allow-prereleases: true
         cache: 'pip'
         cache-dependency-path: 'requirements/*.txt'