Fix some issues reported by lintian

See merge request !1

Changes-By: lintian-brush
Fixes: lintian: priority-extra-is-replaced-by-priority-optional
See-also: https://lintian.debian.org/tags/priority-extra-is-replaced-by-priority-optional.html

Changes-By: lintian-brush
Fixes: lintian: uses-debhelper-compat-file
See-also: https://lintian.debian.org/tags/uses-debhelper-compat-file.html

Changes-By: lintian-brush
Fixes: lintian: package-uses-old-debhelper-compat-version
See-also: https://lintian.debian.org/tags/package-uses-old-debhelper-compat-version.html

Changes-By: lintian-brush
Fixes: lintian: package-lacks-versioned-build-depends-on-debhelper
See-also: https://lintian.debian.org/tags/package-lacks-versioned-build-depends-on-debhelper.html

Changes-By: lintian-brush
Fixes: lintian: build-depends-on-obsolete-package
See-also: https://lintian.debian.org/tags/build-depends-on-obsolete-package.html

Changes-By: lintian-brush
Fixes: lintian: insecure-copyright-format-uri
See-also: https://lintian.debian.org/tags/insecure-copyright-format-uri.html

Changes-By: lintian-brush
Fixes: lintian: trailing-whitespace
See-also: https://lintian.debian.org/tags/trailing-whitespace.html