Fix some issues reported by lintian

Fix some issues reported by lintian

• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking, upstream-metadata-missing-repository)

• Update standards version to 4.5.1, no changes needed. (out-of-date-standards-version)

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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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/xonsh/82f2ecdd-a41d-45f4-b70f-820a53b835bc.

These changes have no impact on the binary debdiff.

You can also view the diffoscope diff (unfiltered).