Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Use secure copyright file specification URI. (insecure-copyright-format-uri)
• debian/copyright: use spaces rather than tabs to start continuation lines. (tab-in-license-text)
• Bump debhelper from deprecated 7 to 13. (package-uses-deprecated-debhelper-compat-version)
• Set debhelper-compat version in Build-Depends. (uses-debhelper-compat-file)
• Change priority extra to priority optional. (priority-extra-is-replaced-by-priority-optional)

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/benchmark.mpi
-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/pi.mpi
-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/wordproblems.mpi

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/benchmark.mpi.gz
-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/pi.mpi.gz
-rw-r--r--  root/root   /usr/share/doc/mathpiper/examples/wordproblems.mpi.gz

Control files: lines which differ (wdiff format)

• Priority: extra optional

You can also view the diffoscope diff (unfiltered).

This merge proposal was created by the Janitor bot, and it will automatically rebase or close this proposal as appropriate when the target branch changes. Any comments you leave here will be read by the Janitor's maintainers.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/mathpiper/f896792e-5755-4009-b4a4-196ff1ceee8b.