Retire _AnyStr

[abelcheung]

The alias _AnyStr has been traditionally used inside lxml-stubs to mean str | bytes, even before being cut off from typeshed. There isn't a big motivation yet to eliminate its usage so far, despite being easily confusable with typing.AnyStr.

Now that typing.AnyStr has been officially deprecated, and types-lxml is trying to update arguments where bytearray is applicable, the time might be appropriate to have _AnyStr gone for good. Even if an alias is desired, something like TextArg makes more sense and less likely mistaken as the deprecated TypeVar.

Tasks ahead

• Search for places where _AnyStr is used for single purpose
  • attribute value comes to mind, especially inside SupportsLaxedItems mapping (233adc9)
• For remaining usages, slowly test for bytearray and replace arguments
• Maybe really create a TextArg alias for this purpose