_md5 ImportError on some Python distributions

[recursiveMake]

There's an error importing claripy on some distributions that don't have _md5. In my case, Python 3.6 on RHEL 7.

This was updated in #226 because the Python implementation of md5 is faster than the openssl version. However, some distro's "FIPS compliant" Pythons don't include it. See here. However, you can still use the openssl implementation via hashlib.

As an alternative, we can try importing _md5 and if that fails fall back to the slower hashlib version.