python-meld3: new package

[thomas.petazzoni@free-electrons.com:
 Fixup indentation in the Config.in file.
 Add 'select BR2_PACKAGE_PYTHON_PYEXPAT' which is needed to get XML
 support in Python.
 Add license information.]

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>