[feat] use OpenSSL backend for scrypt if available
This needs OpenSSL >= 1.1, otherwise it will keep using the scrypt dependency.
We should think about deprecating scrypt as a dependency when we can be sure that the adoption of libssl 1.1 is wide enough. I think that at some point (soledad 0.11 or so) we can drop the scrypt dependency, which was being somehow problematic at times (the _scrypt.so was not appearing when installing with pip, needed workarounds). From that moment on, we can raise an error if an old libssl is found and no scrypt can be imported - leaving that to the user/packager.
In debian stretch and afterwards, you can get that version by installing libssl-dev
- Related: #8472 (closed)