dev-python/ipykernel: adjust dependencies on ipython

dev-python/ipykernel: adjust dependencies on ipython

With ipython6+ dropping py2 and ipython having a circular dependency
with ipykernel and ipyparallel, things get messy and portage's error
messages are unhelpful unless we have a clean break in our dependencies.

With this commit, all <ipykernel-5 depend on <ipython-6 and all
>=ipykernel-5 depend on >=ipython-6. This makes portage's dependency
resolution much happier and results in helpful error messages indicating
what packages need "-python_targets_python2_7".

Bug: https://bugs.gentoo.org/684962
Signed-off-by: Virgil Dupras <vdupras@gentoo.org>
Package-Manager: Portage-2.3.62, Repoman-2.3.11