doc: note pyo3 libpython requirement

Author: somethingelseentirely

CHANGELOG.md

@@ -14,6 +14,7 @@
 - added crate-level examples for weak references and owner downcasting
 - expanded module introduction describing use cases
 - documented rationale for separating `ByteSource` and `ByteOwner`
+- note that the `pyo3` feature requires Python development libraries
 
 ## 0.19.3 - 2025-05-30
 - implemented `Error` for `ViewError`

README.md

@@ -74,6 +74,10 @@ Other optional features provide additional integrations:
 - `zerocopy` – exposes the [`view`](src/view.rs) module for typed zero-copy access and allows using `zerocopy` types as sources.
 - `pyo3` – builds the [`pybytes`](src/pybytes.rs) module to provide Python bindings for `Bytes`.
 
+Enabling the `pyo3` feature requires the Python development headers and libraries
+(for example `libpython3.x`). Running `cargo test --all-features` therefore
+needs these libraries installed; otherwise disable the feature during testing.
+
 ## Examples
 
 - [`examples/quick_start.rs`](examples/quick_start.rs) – the quick start shown above