Link PR numbers in changelog entries?

It would be nice if changelog entries somehow linked back to the corresponding typeshed PRs. Currently, if a user wants more information about a changelog entry, they need to manually copy the PR number and search for it on the typeshed repo. One idea would be to search for the pattern `(#NNNNN)` in new changelog entries and replace it with something like `(python/typeshed#NNNNN)` or `([#NNNNN](https://github.com/python/typeshed/pull/NNNNN))`.