build: run Mypy static analysis as part of "make test"

Fixes: GREG-13

Makefile

+MYPY ?= mypy
 PIP ?= pip -q
 POETRY ?= poetry
 PYTHON ?= python3.9
 VENV ?= venv
 
+mypy = $(MYPY) --config-file mypy.ini
 pip = python -m $(PIP)
 poetry = python -m $(POETRY)
 venv = . $(VENV)/bin/activate &&
@@ -21,6 +23,7 @@ $(VENV)/touchfile:
 
 .PHONY: test
 test: $(VENV)
+	$(venv) $(mypy) -p greg
 	$(venv) python manage.py test
 
 .PHONY: deps