switch from freeze reqs script to pip-tools

instead of alexey's home-grown script, pip-tools offers a quicker, more
efficient and better supported way to freeze requirements.

see prior art here:

https://github.com/alphagov/notifications-admin/pull/3753
https://github.com/alphagov/notifications-ftp/pull/333

Makefile

@@ -54,29 +54,15 @@ generate-version-file: ## Generates the app version file
 	@echo -e "__git_commit__ = \"${GIT_COMMIT}\"\n__time__ = \"${DATE}\"" > ${APP_VERSION_FILE}
 
 .PHONY: test
-test: test-requirements ## Run tests
+test: ## Run tests
 	flake8 .
 	isort --check-only ./app ./tests
 	pytest -n4 --maxfail=10
 
 .PHONY: freeze-requirements
 freeze-requirements: ## Pin all requirements including sub dependencies into requirements.txt
-	rm -rf venv-freeze
-	virtualenv -p python3 venv-freeze
-	$$(pwd)/venv-freeze/bin/pip install -r requirements-app.txt
-	echo '# pyup: ignore file' > requirements.txt
-	echo '# This file is autogenerated. Do not edit it manually.' >> requirements.txt
-	cat requirements-app.txt >> requirements.txt
-	echo '' >> requirements.txt
-	$$(pwd)/venv-freeze/bin/pip freeze -r <(sed '/^--/d' requirements-app.txt) | sed -n '/The following requirements were added by pip freeze/,$$p' >> requirements.txt
-	rm -rf venv-freeze
-
-.PHONY: test-requirements
-test-requirements:
-	@diff requirements-app.txt requirements.txt | grep '<' \
-	    && { echo "requirements.txt doesn't match requirements-app.txt."; \
-	         echo "Run 'make freeze-requirements' to update."; exit 1; } \
-|| { echo "requirements.txt is up to date"; exit 0; }
+	pip install --upgrade pip-tools
+	pip-compile requirements.in
 
 .PHONY: clean
 clean: