Fix for ET's builds-by-cve API new result format

[qixiang]

Errata has changed its builds-by-cve API to return rpms in string list to list of dicts, old format example:

"x86_64": [
    "krb5-libs-1.8.2-3.el6_0.7.i686.rpm",
    "krb5-libs-1.8.2-3.el6_0.7.x86_64.rpm"
]

new format:

"x86_64": [
    {
        "filename": "krb5-libs-1.8.2-3.el6_0.7.i686.rpm",
        "is_signed": true
    },
    {
        "filename": "krb5-libs-1.8.2-3.el6_0.7.x86_64.rpm",
        "is_signed": true
    }
]

This commit updates Freshmaker to accommodate the changes in the result format introduced by the latest ET API.

JIRA: CLOUDWF-10046

[qixiang]

The unit test failures are caused by the updated version of black and other test libraries, which will be fixed in a separated PR.

[cit1zen]

+1 to the hotfix.