dev-libs/gom: bump to 0.4, improve python handling, py3.8

dev-libs/gom: bump to 0.4, improve python handling, py3.8

* Fix for gdk-pixbuf being test-only dep
* Add python-3.8 support; should be fine, as it's just simple gi
  overrides
* Handle python gobject-introspection overrides manually to avoid
  building the whole source tree up to 3 times. The build system
  way is completely broken for us as well with upstream changes in
  0.4 and we'd need to pass the overrides install path manually
  anyhow (upstream changes make it ignore python3.6 being in
  /usr/lib64 and python3.7+ being in /usr/lib properly)
* python isn't needed without introspection; take the chance to also
  add a IUSE=python for this, as we handle it manually anyways and
  don't need upstream support anymore

Package-Manager: Portage-2.3.84, Repoman-2.3.20
Signed-off-by: Mart Raudsepp <leio@gentoo.org>