Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)

• Change priority extra to priority optional. (priority-extra-is-replaced-by-priority-optional)

• Fix broken Vcs URL.

• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

• Remove field Priority on binary package libdxflib-dev that duplicates source. (installable-field-mirrors-source)

This merge proposal was created automatically by the Janitor bot. For more information, including instructions on how to disable these merge proposals, see https://janitor.debian.net/lintian-fixes.

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The bot will automatically update the merge proposal to resolve merge conflicts or close the merge proposal when all changes are applied through other means (e.g. cherry-picks). Updates may take several hours to propagate.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/dxflib/803dcf93-ea01-4d6f-80be-2319c001909c.

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/lib/debug/.build-id/ec/bf2ea6543a4e6e9c5bee701aa662cbbba0c173.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/67/f803f1ac19d87a0768e91f67ab2b20fceae368.debug

Control files of package libdxflib-dev: lines which differ (wdiff format)

• Priority: extra optional

No differences were encountered between the control files of package **libdxflib3**

Control files of package libdxflib3-dbgsym: lines which differ (wdiff format)

• Build-Ids: 67f803f1ac19d87a0768e91f67ab2b20fceae368 ecbf2ea6543a4e6e9c5bee701aa662cbbba0c173

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