There have recently been some changes to the apt mirrors that have affected several Docker images, some of which you might be running in Bitbucket Pipelines. Given you're seeing this error, you're likely affected by this issue. Details of why this is occurring are noted here: https://lists.debian.org/debian-devel-announce/2019/03/msg00006.html
In order to fix this issue, you will need to update the Docker image you are using in your build. For example, if you are using Python:3.5.1, you now need to use a currently supported tag such as 3.5.7 or 3.7.2.
Given this is an issue outside of Pipelines, I'm going to close this ticket. If you have any other questions, or for more information, check out our Community post.