We don't need venv for certbot and can install it directly to the system

Makefile

 all: certbot-timer certbot-service
-	install -v -Dm755 certbot-auto /usr/bin/certbot-auto
+	pip3 install certbot
 	install -v -m755 -d /etc/letsencrypt
-	USE_PYTHON_3=1 certbot-auto --install-only --no-bootstrap --no-self-upgrade
 
 	@echo "$$CERTBOT_TIMER" > /lib/systemd/system/certbot.timer
 	@echo "$$CERTBOT_SERVICE" > /lib/systemd/system/certbot.service
@@ -28,6 +27,6 @@ Description=Renew certificates
 
 [Service]
 Type=simple
-ExecStart=/usr/bin/certbot-auto --no-self-upgrade renew
+ExecStart=/usr/bin/certbot renew
 endef
 export CERTBOT_SERVICE