Added unstable feature for stream conversions

Author: Andrew J Westlake

Cargo.toml

@@ -21,6 +21,7 @@ async-std-runtime = ["async-std"]
 attributes = ["pyo3-asyncio-macros"]
 testing = ["clap"]
 tokio-runtime = ["tokio"]
+unstable-streams = []
 default = []
 
 [package.metadata.docs.rs]
@@ -109,6 +110,7 @@ once_cell = "1.5"
 pin-project-lite = "0.2"
 pyo3 = "0.16"
 pyo3-asyncio-macros = { path = "pyo3-asyncio-macros", version = "=0.16.0", optional = true }
+stability = "0.1"
 
 [dev-dependencies]
 pyo3 = { version = "0.16", features = ["macros"] }