Add a pylint plugin that enforces:
- There is no bare `import dns` statement.
- All `dns.<module>` used are explicitly imported.
- There are no unused `dns.<module>` imports.
Fix all the imports to conform with this check.
Importing pytest fixture trips up static analysis tools, so move
default_algorithm to conftest.py and use it instead of os.environ
accesses in various system tests.
For use outside test function, use Algorithm.default().
This is a follow-up from the test cases that were added long time ago
that cover migrating to dnssec-policy.
The test covers legacy keys that do not have the SyncPublish timing
metadata set.
Various test cases where legacy keys exist and are used to migrate to
dnssec-policy. These once existed as shell script but were removed
because auto-dnssec was removed. But the test cases are still useful
if one wants to migrate to dnssec-policy with such legacy keys.
The tests were previously removed in commit
906dd57b68 as part of MR !8085.
