date: accept vertical tab and form feed as whitespace

[wtcpython]

Fixes #13864.

GNU date accepts vertical tab and form feed as whitespace between items, we rejected them. The fix landed upstream in parse_datetime, so track its git main until the next release, and add tests.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-parameters. tests/timeout/timeout-parameters is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/misc/io-errors (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/pipe-f is now passing!