Fix flake8 violations for all files in the asv/plugins/ directory

[LucyJimenez]

Closes #1044

Clean out some of the files to follow Flake8 standards on the asv/plugins folder.

[datapythonista]

Thanks @LucyJimenez. The noqa message is a bit confusing, not sure which previous step you are referring to. The import is needed even if not used, since the goal is to check if the library exists.

[LucyJimenez]

Thanks @LucyJimenez. The noqa message is a bit confusing, not sure which previous step you are referring to. The import is needed even if not used, since the goal is to check if the library exists.

Thanks @datapythonista. It is a nice recommendation to take in mind for the next messages :)