This reverts commit f09ab8889148c406551dea1643966e3331fde4aa, reversing
changes made to effc761e0ee4ea7ffb77f275b5ed650a7098d6f8.
This is being temporarily reverted to make it easier to release an
urgent fix for v10.15.1.
This PR adds `snaps` under Flask build flags to the extension. This branch is mostly equivalent to the current production version of Flask, excepting some bug fixes and tweaks.
Closes#11626