Update Towncrier config in pyproject.toml

Situation

Current towncrier configuration uses a deprecated format (see Towncrier's configuration reference)

  [[tool.towncrier.type]]
  directory = "trivial"
  name = "Trivial/Internal Changes"
  showcontent = true

Use the new format:

  [tool.towncrier.fragment.trivial]
  name = "Trivial/Internal Changes"
  showcontent = true

(array of toml tables is deprecated)