Resolve "CI broken due to defunct jessie-backports"

Closes #61

See merge request !41

Jessie support removed as jessie-backports was shut down. Also removed support for python2 since modern systems either have python3, or can install python3.

Resolve "Don't remove staple files but zero them"

Closes #56

See merge request !40

Don't delete staple files for invalid certificates, instead zero the file so HAProxy will still serve staples once the certificates are valid again. And fix some PEP8 and pylint issues.