Remove CircleCI

CircleCI tests currently seem fail consistently. GitHub actions
introduced in #136 are providing the same coverage (and more), have been
running stable for over a year and are easier to maintain. Thus, remove
Circle CI integration.

Ref. #136 (comment)

Allow to specify CPU usage period as duration string

The current implementation assumes the given duration in seconds.
Instead, also allow to specify a duration in the format as expected by
`time.ParseDuration` and use that if found. If a plain integer is
specified, it is still parsed as a duration in seconds.