Warn when a string is passed as choices to OneOf

[ChrisJr404]

Closes #2074.

A string is a valid iterable, so it "works" as choices for OneOf — but it's almost always a mistake. The motivating case from the issue:

validate.OneOf("one", "two")   # oops: "one" is choices, "two" is labels

"one" in "one" is True, so the value "one" validates, and each character of the string is treated as a separate choice in the error text and in .options(). This is easy to hit and hard to spot.

This PR emits a UserWarning (at stacklevel=2, so it points at the caller) when a plain OneOf is constructed with a str for choices. Wrapping the string in a list silences it, as @deckar01 suggested on the issue:

validate.OneOf(list("abc"))   # no warning

ContainsOnly subclasses OneOf but legitimately accepts a string as a character set (that usage is covered by the existing test suite), so the warning is scoped to exact OneOf instances via type(self) is OneOf and ContainsOnly is unaffected. Behaviour is otherwise unchanged — string choices still validate exactly as before.

• Added test_oneof_string_choices_warns covering the warning, the list-wrapping silence path, and that ContainsOnly does not warn.
• Updated the existing string-based assertions in test_oneof to expect the warning.
• Added a changelog entry and an AUTHORS.rst line.