Ignore locale when formatting dates
    
    Dates in RPM changelogs must use a set format with abbreviated English
    names of weekdays and months. Previously, we used datetime.strftime()
    which always honours locale settings, so when run under e.g. `fedpkg
    local` with a locale that uses another format or language, this
    generated invalid RPM changelog entries.
    
    Because the CLI didn't initialize locale, this never caught anyone's eye
    before.
    
    Fixes: #104
    
    Signed-off-by: Nils Philippsen <nils@redhat.com>