tests: flake8 --exclude must be combined in the same value
tests: flake8 --exclude must be combined in the same value

There cannot be multiple --exclude options, the last one will take
precedence.

ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,heartsucker/securedrop