Change the way to detect native pbdkf2 support.

`hashlib.pbdkf2_hmac` was introduced in version 2.7.8.

The built-in `hashlib.pbdkf2_hmac` is much faster.

Here is the benchmark result for 1000 times `generate_password_hash`:

1. werkzeug way: 9.906912s
2. native way: 4.98566