Reproduction for Renovate issue 31876.
This can't be reproduced directly because according to https://docs.renovatebot.com/configuration-options/#postupgradetasks, only self hosted renovatebot servers can use postUpgradeTasks. The server must be configured with https://docs.renovatebot.com/self-hosted-configuration/#allowedpostupgradecommands. However, this renovate.json should work, if the server running against the repo has allowedPostUpgradeCommands: [".*echo.*$"], configured.
Renovate does not run the postUpgradeTasks after requirements.in changes.
Renovate should run the postUpgradeTasks after requirements.in changes. The task should generate/update the lock file.