sys-cluster/singularity: bump to 3.5.3

sys-cluster/singularity: bump to 3.5.3

Invoking maintainer time-out on this one, on the one hand we have been
LONG overdue for a bump to major version 3 and on the other 2.6.2 has
apparently got problems with squashfs on newer kernels.

Discussion of dependencies, given some of them are not obvious and
official "install from source" documentation seems to include packages
which are not in fact needed:
 - sys-libs/libseccomp - looked for by mconfig, moreover at least on my
   system singularity binaries end up linked *dynamically* against it;
 - virtual/pkgconfig - used by mconfig while looking for libseccomp;
 - go-1.13+ - this version number is requested in mconfig;
 - app-crypt/gpgme - NOT looked for by mconfig but Singularity does use
   gpgme bindings for Go;
 - dev-libs/openssl - looked for by mconfig;
 - sys-apps/util-linux - libuuid and its header files looked for by mconfig;
 - sys-fs/cryptsetup - looked for by mconfig;
 - sys-fs/squashfs-tools - various tools from this package (e.g. unsquash)
   are invoked at run time, that said I have experimentally confirmed that
   Singularity builds fine without it.

Closes: https://bugs.gentoo.org/694048
Signed-off-by: Marek Szuba <marecki@gentoo.org>