license.py: avoid deprecated ast.Str

license.py: avoid deprecated ast.Str

* it's deprecated since python-3.12 and removed in 3.14 causing:

openembedded-core/meta/lib/oe/license.py', lineno: 176, function: visit
     0172:
     0173:        LicenseVisitor.__init__(self)
     0174:
     0175:    def visit(self, node):
 *** 0176:        if isinstance(node, ast.Str):
     0177:            lic = node.s
     0178:
     0179:            if license_ok(self._canonical_license(self._d, lic),
     0180:                    self._dont_want_licenses) == True:
Exception: AttributeError: module 'ast' has no attribute 'Str'

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>