This fixes the following warnings:
```
.../sphinx_gallery/py_source_parser.py:101: DeprecationWarning: ast.Str is
  deprecated and will be removed in Python 3.14; use ast.Constant
  instead
    and isinstance(node.body[0].value, ast.Str)

.../sphinx_gallery/py_source_parser.py:113: DeprecationWarning:
  Attribute s is deprecated and will be removed in Python 3.14; use
  value instead
    if len(node.body[0].value.s) and node.body[0].value.s[0] == "\n":
```

`ast.Str` was deprecated in 3.8 (and returned an `ast.Constant` anyway),
which is the minimum requirement here. I can't find `s` or `value`, so
I'm not sure when they were deprecated exactly.