Simplify compare functions for sorting

[mielvds]

This PR removes some of 'old-style' python code and fixes linter issues:

• remove the need for the cmp() function wrt to sorting
• use isinstance() and is to do type checking (instead of for instance type(x) == bool)
• change not X in Y to X not in Y
• always raise specific error

[davidlehn]

• Thanks for cleanup.
• I think much this style was originally due to manual js to py conversion.