fix log-level arg parsing

[blampe]

Fixes

$ curd -l DEBUG
usage: curd [-h] [-l {WARN,ERROR,DEBUG,INFO,WARNING,CRITICAL}]
            [--log-file LOG_FILE] [--log-name LOG_NAME] [-q] [-v]
            {install,uninstall,freeze} ...
curd: error: argument -l/--log-level: invalid upper value: 'DEBUG'

[clarete]

Hi @blampe, thanks so much for the patch. I think the tests broke for python3, could you work on that too? Also, just throwing an idea, what about using this opportunity to accept lower values? Like something like this: -ldebug