Fixed Python 2 regression in 6dbe56ed.

Thanks Simon Charette for the implementation idea.

Thanks Shivang Bharadwaj for the initial patch.

Thanks Carl Meyer and Tim Graham for contributing to the patch.

This allows running foreign key data and schema altering operations in the
same migration on PostgreSQL.

Thanks Tim for review.

Fixed #26543 -- Prevented "confirm form submission" browser prompt when reloading after an admin actions failure.

The "needs to have a value for field" messages are incorrect and
reference nonexistent fields since the commit in which they were
introduced (refs #18823).

starttls(), login(), and other connection methods already call
the method as needed.

Thanks Adam Chainz for review and guidance.

Thanks Jaap Roes for the initial patch.