archivers/py-borgbackup: use fixed xxhash instead of avoiding it

This reverts commit 9db903a8 because
it does not fix the situation for lack of a PORTREVISION bump.
Let us use the opportunity to fix the real bug instead.

Now with xxhash fixed as of 0.8.1_2, drop 9db903a8, and
bump PORTREVISION so we flush out the old garbage packages.

A simple web search would have brought up that static_assert() is
a C11 convenience macro defined in <assert.h>, and a detailed
search would have turned up that this had been fixed in xxhash
before.

While here, add a smoke test to post-install.

Proper fix was
Reported by:	Thierry Dussuet <thierry.dussuet@protonmail.com>
PR:		265060
MFH:		2022Q3