[3.9] gh-127359: Pin Tcl/Tk to 8 (8.6) for testing macOS (GH-127365) (#127409)

Author: hugovk

.github/workflows/build.yml

@@ -158,7 +158,10 @@ jobs:
     steps:
     - uses: actions/checkout@v4
     - name: Install Homebrew dependencies
-      run: brew install pkg-config [email protected] xz gdbm tcl-tk
+      run: |
+        brew install pkg-config [email protected] xz gdbm tcl-tk@8
+        # Because alternate versions are not symlinked into place by default:
+        brew link tcl-tk@8
     - name: Configure CPython
       run: |
         CC=clang \