policy and pylib/packet cmac/160 bit hmac support

[James Browning]

After looking at devel/HACKING, I do not see a policy on including
external python modules.

The came up because I have a merge request (!1044), which adds support
for RIPEMD160, SHA-1, and AES128CMAC. The CMAC implementation currently
requires the pycryptodome[1] module.

If external modules are not allowed, then eventually someone will need
to implement a substitute.

[1] https://pycryptodome.readthedocs.io/
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