mozcoreconf-v6.eclass: add still required Python 2.7 dependency

mozcoreconf-v6.eclass: add still required Python 2.7 dependency

In commit 9d973365a74e58731b62caae87d4ad521c90d947 and
af9298355290622e1ab2ec5253550e6b8196722a Python 3 support was added
and PYTHON_COMPAT was moved from eclass to ebuild as requested by QA.

However, Python 2 dependency, which is still required, was lost.

This commit will bring back Python 2 dependency.

Note that we use python-any-r1_pkg_setup in moz_pkgsetup to initialize
Python 3 via PYTHON_COMPAT but we will switch back to Python 2 manually
using python_export later.

Closes: https://bugs.gentoo.org/679582
Signed-off-by: Thomas Deutschmann <whissi@gentoo.org>