pyassuan-9999.ebuild: Bump to EAPI 6, git-r3, and distutils-r1

Also claim compatibility with Python 3.3 through 3.6 to match upstream
since [1] and drop nose for testing to match upstream since [2].

[1]: https://github.com/wking/pyassuan/commit/7bdffe153e0999625a9e874ce68c5fcf43c3eac8
[2]: https://github.com/wking/pyassuan/commit/1c19d389bbf5894a0bf451e2fc9ddace4317661a

ChangeLog: Drop this optional file

Users who need a history can get it from the Git repository, and the
file is optional since at least 2006 [1].

[1]: https://gitweb.gentoo.org/proj/pms.git/commit/?id=81f522241733c87bcfca90b554082a2d0782cd8f

Manifest: Remove this file

Move to thin manifests [1,2] now that Gentoo supports them.

[1]: https://wiki.gentoo.org/wiki/Repository_format/package/Manifest#Thin_Manifest
[2]: https://wiki.gentoo.org/wiki/Repository_format/metadata/layout.conf#thin-manifests

dev-python/pyassuan: require Python 3.3 for socket.sendmsg() and .recvmsg().

Add dev-python/pyassuan package.