Rollback to pyo3 = { version = "0.16.6", features = ["abi3"] }

In order to avoid `ImportError: PyO3 modules compiled for CPython 3.8 or
older may only be initialized once per interpreter process` introduced
since pyo3 >= 0.17.0, which only allow each #[pymodule] to be
initialized once, this PR temporarily rollback to pyo3 = 0.16.6.

It also remove the use of `PyUserWarning` which introduced since pyo3 >
0.18.0.

See PyO3/pyo3@78ba70d
See PyO3/pyo3@1d20f2a

Fixes pyca#694

Signed-off-by: Wong Hoi Sing Edison <hswong3i@pantarei-design.com>

src/_bcrypt/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 publish = false
 
 [dependencies]
-pyo3 = { version = "0.20.1", features = ["abi3"] }
+pyo3 = { version = "0.16.6", features = ["abi3"] }
 bcrypt = "0.15"
 bcrypt-pbkdf = "0.10.0"
 base64 = "0.21.5"

src/_bcrypt/src/lib.rs

@@ -151,18 +151,6 @@ fn kdf<'p>(
         ));
     }
 
-    if rounds < 50 && !ignore_few_rounds {
-        // They probably think bcrypt.kdf()'s rounds parameter is logarithmic,
-        // expecting this value to be slow enough (it probably would be if this
-        // were bcrypt). Emit a warning.
-        pyo3::PyErr::warn(
-            py,
-            pyo3::exceptions::PyUserWarning::type_object(py),
-            &format!("Warning: bcrypt.kdf() called with only {rounds} round(s). This few is not secure: the parameter is linear, like PBKDF2."),
-            3
-        )?;
-    }
-
     pyo3::types::PyBytes::new_with(py, desired_key_bytes, |output| {
         py.allow_threads(|| {
             bcrypt_pbkdf::bcrypt_pbkdf(password, salt, rounds, output).unwrap();