[oe] asn1crypto dependency missing for python-cryptography

[Satya Bodduluri]

In versions of python-cryptography >=1.8.x, it includes a dependency for
asn1crypto and removes a dependency on pyasn1 as can be seen with this
commit:
https://github.com/pyca/cryptography/commit/0e6a129724b707ebf79149376251e85fad550414.
As
far as I can tell, asn1crypto isn't included in
meta-openembedded/meta-python.

I've managed to work around this by using an older version of
python-cryptography, but I just wanted to make note of this as it makes
portions of the newer versions of the cryptography library (the master
branch of meta-openembedded is using version 2.0.3 of python-cryptography)
unusable when it can't import asn1crypto. I'm not sure when I'll have time
to create a patch, but I can take a stab at it if no one else beats me.

Thanks,
Satya Bodduluri