replace ast.Str with ast.Constant to avoid deprecation warnings in py…

…thon 3.12

jaxtyping/_import_hook.py

@@ -144,7 +144,7 @@ def visit_Module(self, node: ast.Module):
  for i, child in enumerate(node.body):
  if isinstance(child, ast.ImportFrom) and child.module == "__future__":
  continue
- elif isinstance(child, ast.Expr) and isinstance(child.value, ast.Str):
+ elif isinstance(child, ast.Expr) and isinstance(child.value, ast.Constant):
  continue # module docstring
  else:
  node.body.insert(i, ast.Import(names=[ast.alias("jaxtyping", None)]))