Add vendored packages back

lib/spack/external/_vendoring/attr/converters.pyi

@@ -0,0 +1,13 @@
+from typing import Callable, Optional, TypeVar, overload
+
+from . import _ConverterType
+
+_T = TypeVar("_T")
+
+def pipe(*validators: _ConverterType) -> _ConverterType: ...
+def optional(converter: _ConverterType) -> _ConverterType: ...
+@overload
+def default_if_none(default: _T) -> _ConverterType: ...
+@overload
+def default_if_none(*, factory: Callable[[], _T]) -> _ConverterType: ...
+def to_bool(val: str) -> bool: ...