Remove Python 3.12-dev CI config

Python 3.12 is now released, so this is what "3.x" uses.

.github/workflows/ci.yaml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         platform: [ubuntu-latest, windows-latest]
-        python-version: ['3.7', '3.8', '3.9', '3.x', '3.12-dev']
+        python-version: ['3.7', '3.8', '3.9', '3.x']
 
     env:
       PY_COLORS: 1
@@ -45,7 +45,7 @@ jobs:
           sudo apt install ffmpeg  # For replaygain
 
       - name: Test older Python versions with tox
-        if: matrix.python-version != '3.x' && matrix.python-version != '3.12-dev'
+        if: matrix.python-version != '3.x'
         run: |
           tox -e py-test
 
@@ -63,15 +63,6 @@ jobs:
         run: |
           tox -vv -e py-mypy
 
-      - name: Test nightly Python version with tox
-        if: matrix.python-version == '3.12-dev'
-        # continue-on-error is not ideal since it doesn't give a visible
-        # warning, but there doesn't seem to be anything better:
-        # https://github.com/actions/toolkit/issues/399
-        continue-on-error: true
-        run: |
-          tox -e py-test
-
       - name: Upload code coverage
         if: matrix.python-version == '3.x'
         run: |