chore: Pipfile's dev-packages omits isort, which make isort and test_isort_clean both need #810

Describe the bug

Pipfile's [dev-packages] lists mypy and pylint but not isort, while make isort and tests/project/test_isort_clean.py both depend on it. So a pipenv install --dev environment cannot run the isort recipe or that test, and the test skips rather than fails — invisibly.

Pipfile:48  mypy = "*"
Pipfile:49  pylint = "*"
            (no isort entry)

Expected behavior

isort in [dev-packages] alongside mypy and pylint, so the three static-analysis tools the Makefile drives are all installable the same way.

Additional context

#786 already put isort on the test extra in pyproject.toml and gated test_isort_clean.py visibly, and pyproject.toml:267-273 carries the reasoning — that the test is "the only guard on make isort's four recipe lines". Pipfile was not updated to match, so the two dependency declarations disagree about a tool the Makefile needs.

Check whether the same gap exists for any other Makefile-driven tool while here — compare Pipfile's [dev-packages] against every executable the Makefile invokes, and report the comparison rather than only fixing isort.

Trap: the two files serve different audiences (pyproject.toml extras for consumers of the package, Pipfile for this repo's dev environment), so the fix is to add it to Pipfile, not to remove it from either. Do not "unify" them.

Related: #786, #719.