feat: issue warning if pyproject.toml was updated without pkg update

Makefile

@@ -5,6 +5,7 @@
 # whether this current run of `make` requires a Python virtual environment.
 NEED_VENV := $(or \
   $(findstring setup,$(MAKECMDGOALS)), \
+  $(findstring upgrade-quiet,$(MAKECMDGOALS)), \
   $(findstring upgrade,$(MAKECMDGOALS)), \
   $(findstring requirements,$(MAKECMDGOALS)), \
   $(findstring all,$(MAKECMDGOALS)), \
@@ -26,6 +27,13 @@ else
   endif
 endif
 
+# If the project configuration file has been updated (package deps or
+# otherwise) then warn the user and suggest resolving the conflict.
+ifeq ($(shell test pyproject.toml -nt .venv/upgraded-on; echo $$?),0)
+  $(warning pyproject.toml was updated, consider `make upgrade` if your packages have changed)
+  $(warning If this is not correct then run `make upgrade-quiet`)
+endif
+
 # Create a virtual environment, either for Python3.10 (default) or using
 # the Python interpreter specified in the PYTHON environment variable.
 .PHONY: venv
@@ -57,10 +65,12 @@ upgrade: .venv/upgraded-on
 .venv/upgraded-on: pyproject.toml
 	python -m pip install --upgrade pip
 	python -m pip install --upgrade --upgrade-strategy eager --editable .[hooks,dev,test,docs]
-	echo "Automatically generated by Python Package Makefile on `date --rfc-3339=seconds`." > .venv/upgraded-on
+	make upgrade-quiet
 force-upgrade:
 	rm -f .venv/upgraded-on
-	$(MAKE) upgrade
+	make upgrade
+upgrade-quiet:
+	echo "Automatically generated by Python Package Makefile on `date --rfc-3339=seconds`." > .venv/upgraded-on
 
 # Generate a requirements.txt file containing version and integrity
 # hashes for all packages currently installed in the virtual environment.
@@ -111,7 +121,7 @@ dist/package-$(PACKAGE_VERSION).tar.gz: check test
 .PHONY: docs
 docs: docs/_build/html/index.html
 docs/_build/html/index.html: check test
-	$(MAKE) -C docs/ html
+	make -C docs/ html
 
 # Clean test caches and remove build artifacts.
 .PHONY: dist-clean clean