Rename `at` aliases to `pt`

[ricardoV94]

Description

As in other shallow refactor PRs we did, may make sense to sign commits as pymc-bot and pymc-devs: https://github.com/pymc-devs/pymc/blob/6bfbff508b80e588cc03711798b4e2577ad46449/.github/workflows/autoupdate-pre-commit-config.yml#L32-L33