Add changelog entry detailing how much further impl FromPyObject for …

…Vec<T> was relaxed.

newsfragments/2632.changed.md

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+`impl FromPyObject for Vec<T>` will accept any Python object that can be turned into an iterator via Python's built-in `iter` function. It will also not reject `str` anymore which can be iterated as a sequence of `str` objects, i.e. [#2500](https://github.com/PyO3/pyo3/pull/2500) was reverted. Please use a type like
+
+```rust
+#[derive(FromPyObject)]
+pub enum OneOrMany<'py> {
+    One(&'py PyString),
+    Many(Vec<&'py PyString>),
+}
+```
+
+if you would like to work around this edge case for callers of your API.