package/gobject-introspection: depend on python3

package/gobject-introspection: depend on python3

Currently, the Config.in file has the line:
select BR2_PACKAGE_PYTHON3 if !BR2_PACKAGE_PYTHON

This line is incorrect as gobject-introspection does not support python2.
Instead, remove the select line and make python3 a dependency with a new
message that explains that gobject-introspection requires python3.

Signed-off-by: Adam Duskett <Aduskett@gmail.com>
[yann.morin.1998@free.fr:
  - move the explanations from the commit log to the code
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>