chore: add isort to Pipfile's dev-packages so make isort resolves it

Pipfile

@@ -45,6 +45,7 @@ beautifulsoup4 = {extras = ["html5lib"],version = "*"}
 requests = {extras = ["socks"],version = "*"}
 autopep8 = "*"
 bandit = "*"
+isort = "*"
 mypy = "*"
 pylint = "*"
 ipython = "*"

tests/project/test_isort_clean.py

@@ -145,10 +145,12 @@
 below take about 1.5s together, the "costs nothing per leg" category the test
 job's own install comment puts ``DPKT`` in rather than the ``Scapy`` one.
 
-``isort`` remains absent from ``Pipfile``, which is what ``make isort``'s own
-``pipenv run`` prefix resolves against -- so the target still depends on a
-contributor having isort on ``PATH``. Out of scope here: this module is reached
-by pytest, not by ``make``.
+``isort`` is in ``Pipfile``'s ``[dev-packages]`` too, as of #810 -- that being
+what ``make isort``'s own ``pipenv run`` prefix resolves against, so the target
+no longer depends on a contributor happening to have isort on ``PATH``. The two
+declarations are additive rather than duplicate: the ``pyproject.toml`` extra
+serves consumers installing the package, ``Pipfile`` serves this repository's
+own dev environment, and ``make isort`` reads only the latter.
 
 """