config: use old debseries module for now

Nix CI is using PyPI apkg for bootstrap, which doesn't know deb_series
yet.

Leave the old debseries module in config for some time.

distro/config/apkg.toml

@@ -12,7 +12,8 @@ version_script = "scripts/upstream-version.py"
 
 # optional - custom template variables deb_series and deb_codename
 [[template.variables]]
-python_module = "apkg.templatevars.deb_series"
+# TODO: rename to deb_series after 0.6 release
+python_module = "apkg.templatevars.debseries"
 
 [apkg]
 # recommended - apkg compat level