For 3.7.1rc1 and 3.6.7rc1 we used a pre-release development
snapshot of Tk 8.6 to pick up some post-8.6.8 fixes for macOS.
But the snapshot introduced at least one regression (bpo-34927).
For rc2, revert to using the standard release 8.6.8 for now.
This reverts commit d9cfe5ed.