Fix failures in Debian packaging

[callahad]

Debian package builds were failing for two reasons:

1. Python versions prior to 3.7 throw exceptions when attempting to print Unicode characters under a "C" locale.

2. We depended on dh-systemd which no longer exists in Debian Bullseye, but is necessary in Ubuntu Xenial.

Fixing the first issue is as easy as setting LANG="C.UTF-8" in the build environment. (See https://bugs.python.org/issue19846)

Fixing the second issue requires attempting to install dh-systemd, but ignoring failures when it does not exist.

Specifically, the dh-systemd package was merged with the main debhelper package as of debhelper (>= 9.20160709)` and a transitional package was left in its place.

That transitional package was removed in Debian Bullseye after being present in both Stretch and Buster. Thus, on the Debian side, just removing the dependency on dh-systemd would be sufficient.

However, Ubuntu Xenial has an old debhelper version, and thus actually needs dh-systemd. So we still try to install it on all distros, but ignore when that fails.

Once we drop support for Xenial (March 2021?) we can completely remove anything referencing dh-systemd.

Fixes #9073
Fixes #9076

[callahad]

Cancelled CI since we don't test the Debian build scripts in CI.

Currently building all debs locally to ensure things work. Stretch completed successfully, and I expect no further failures.

[callahad]

Per GitHub comment above, this basically says "try to install dh-systemd, but it's OK if it fails or doesn't exist."

dh-systemd exists as a dummy transitional package on all releases except Ubuntu xenial (sans backports), where we actually do need it. So we need to try to install it. However, the transitional package was removed in Debian Bullseye, so we have to gracefully continue even if the package does not exist.

...all this goes away when we drop Xenial support in ~2 months.

[richvdh]

thanks for poking at this. can you add some context to the PR about what broke and why, and how this fixes it?