sci-mathematics/flint: new EAPI=7 revision.

sci-mathematics/flint: new EAPI=7 revision.

This new revision is based off François Bissey's ebuild in the
sage-on-gentoo overlay, which has accrued some fixes not present in
the ::gentoo version. Of note, it fixes an open build bug, and an
unreported issue where 32-bit libraries are detected on a 64-bit
system.

The update to EAPI=7 required only moving the LaTeX dependencies to
BDEPEND. In the process, I updated the LICENSE to LGPL-2.1+ because
upstream has done the same. I also looked long and hard for a reason
why we have RESTRICT=mirror in the ebuild. Finding none, I have
removed the restriction.

Closes: https://bugs.gentoo.org/470732
Closes: https://bugs.gentoo.org/688674
Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Michael Orlitzky <mjo@gentoo.org>