Use Debian "sid" for pylint and mypy jobs to get recent dnspython

The base image tends to have a rather old dnspython version and when
used with pylint and mypy it produces errors about newer dnspython
features the old version does not know about.

    $ mypy "bin/tests/system/isctest/"
    bin/tests/system/isctest/query.py:55: error: Unexpected keyword argument "verify" for "tls"  [call-arg]
    /usr/lib/python3/dist-packages/dns/query.py:958: note: "tls" defined here

    $ pylint --rcfile $CI_PROJECT_DIR/.pylintrc --disable=wrong-import-position $(git ls-files 'bin/tests/system/*.py' | grep -vE 'ans\.py')
    ************* Module isctest.query
    bin/tests/system/isctest/query.py:55:11: E1123: Unexpected keyword argument 'verify' in function call (unexpected-keyword-arg)

.gitlab-ci.yml

@@ -577,7 +577,9 @@ coccinelle:
     - if test "$(git status --porcelain | grep -Ev '\?\?' | wc -l)" -gt "0"; then git status --short; exit 1; fi
 
 pylint:
-  <<: *precheck_job
+  <<: *default_triggering_rules
+  <<: *debian_sid_amd64_image
+  stage: precheck
   needs: []
   variables:
     PYTHONPATH: "${CI_PROJECT_DIR}/bin/tests/system"
@@ -630,7 +632,9 @@ checkbashisms:
     - checkbashisms $(find . -path './.git' -prune -o -type f -exec sh -c 'head -n 1 "{}" | grep -qsF "#!/bin/sh"' \; -print)
 
 mypy:
-  <<: *precheck_job
+  <<: *default_triggering_rules
+  <<: *debian_sid_amd64_image
+  stage: precheck
   script:
     - mypy "bin/tests/system/isctest/"