* Add extra-requirements.txt
In grand schema of things we now have `requirements.txt`
that list mandatory dependencies, then we have
`extra-requirements.txt` for optional dependencies enabling
all pysnmp features and finally `devel-requirements.txt`
for things required solely for testing.
Strangely, Travis pulls pysnmpcrypto which then pulls
Cryptography while according to pysnmpcrypyo requirement
it should really pull pycryptodomex (which is does in its
own CI job)
Ilya Etingof