Install local package in editable mode (#454)

Makefile

@@ -14,7 +14,7 @@ $(VENV)/bin/python:
 install: $(INSTALL_STAMP) pyproject.toml requirements.txt
 $(INSTALL_STAMP): $(VENV)/bin/python pyproject.toml requirements.txt
 	$(VENV)/bin/pip install -r requirements.txt
-	$(VENV)/bin/pip install ".[dev]"
+	$(VENV)/bin/pip install -e ".[dev]"
 	touch $(INSTALL_STAMP)
 
 lint: install