Fix some issues reported by lintian

Fix some issues reported by lintian

• Update watch file format version to 4. (older-debian-watch-file-standard)
• Remove 2 obsolete maintscript entries in 1 files.
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Repository-Browse. (upstream-metadata-file-is-missing)
• Update standards version to 4.6.2, no changes needed. (out-of-date-standards-version)

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 first set of .debs but not in second

-rwxr-xr-x  root/root   DEBIAN/postrm
-rwxr-xr-x  root/root   DEBIAN/preinst

No differences were encountered in the control files

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/mitmproxy/acf50620-6140-4b91-833a-52c3910d6dc3.