use pep8 standard

hi

pep8 is best Python coding standard I think.

If you don't like it, just close the PR.

Thanks. :smiley: 
PEP-8 contains a clause we liberally apply in this instance: "Blank lines may be omitted between a bunch of related one-liners (e.g. a set of dummy implementations)."

This group of exceptions mainly serves as "type tags", to allow consumers to match different failure classes. As the exception classes themselves are trivial stubs, the additional whitespace does not add much. To the contrary, the lack of additional blank lines visually clusters those exceptions and thereby hints at their relatedness.