this commit adds refurb as a Python linter, together with some tests
and documentation. it should fix issue: #4362
refurb repo: https://github.com/dosisod/refurb
`checkmake` by default checks config file "in the same folder it's
executed in" unless `--config` option is set.
This commit allows setting the `--config` option with an option
variable (with documentation updated).
Yining