Run mypy checks on Python helpers in GitLab CI

Ensure the type hints provided in helper code for Python-based system
tests are correct by continuously checking them using mypy in GitLab CI.
Check bin/tests/system/isctest.py exclusively for the time being because
it is the only Python file in the source tree which uses static typing
at the moment and working around the issues reported by mypy for other
(non-statically-typed) Python files present in the source tree would be
cumbersome.

(cherry picked from commit 989d22473d)

.gitlab-ci.yml

@@ -644,6 +644,11 @@ checkbashisms:
   script:
     - checkbashisms $(find . -path './.git' -prune -o -type f -exec sh -c 'head -n 1 "{}" | grep -qsF "#!/bin/sh"' \; -print)
 
+mypy:
+  <<: *precheck_job
+  script:
+    - mypy "bin/tests/system/isctest/"
+
 tarball-create:
   stage: precheck
   <<: *base_image