Add Makefile to update requirements files

requirements/Makefile

+# Makefile for updating BuildStream's requirements files.
+#
+
+REQUIREMENTS_IN := $(wildcard *.in)
+REQUIREMENTS_TXT := $(REQUIREMENTS_IN:.in=.txt)
+PYTHON := python3
+VENV := $(PYTHON) -m venv
+
+VENV_PIP = $(VENVDIR)/bin/pip
+
+
+.PHONY: all
+
+all: $(REQUIREMENTS_TXT)
+
+%.txt: %.in
+	$(eval VENVDIR := $(shell mktemp -d $$TMPDIR/bst-venv.XXXXXX))
+	$(VENV) $(VENVDIR)
+	$(VENV_PIP) install -r $^
+	$(VENV_PIP) freeze -r $^ > $@
+	rm -rf $(VENVDIR)