Start testing Python 3.15 Alpha (#7195)

* Add optional 3.15-dev runners to test matrix

* Set PYO3_USE_ABI3_FORWARD_COMPATIBILITY for 3.15 builds

Author: nateprewitt

.github/workflows/run-tests.yml

@@ -8,11 +8,12 @@ permissions:
 jobs:
   build:
     runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ matrix.python-version == '3.15-dev' }}
     timeout-minutes: 10
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14-dev", "pypy-3.11"]
+        python-version: ["3.9", "3.10", "3.11", "3.12", "3.13", "3.14", "3.15-dev", "pypy-3.11"]
         os: [ubuntu-22.04, macOS-latest, windows-latest]
         # Pypy-3.11 can't install openssl-sys with rust
         # which prevents us from testing in GHA.
@@ -28,6 +29,8 @@ jobs:
         cache: 'pip'
         allow-prereleases: true
     - name: Install dependencies
+      env:
+        PYO3_USE_ABI3_FORWARD_COMPATIBILITY: ${{ matrix.python-version == '3.15-dev' && '1' || '' }}
       run: |
         make
     - name: Run tests