lang/spidermonkey140: fix build with Python 3.14+ (+)

AttributeError: module 'ast' has no attribute 'Str'

Python have deprecated ast.Str, ast.Num, ast.Bytes since 3.8 and dropped they out in 3.14
Use ast.Constant instead

Reported by: bulk -t
Approved by: portmgr blanket (fix build)