Pynguin v0.16.0

• Refactor the assertion generation. This unifies the SIMPLE and the
  MUTATION_ANALYSIS approaches. Furthermore, Pynguin now uses the
  MUTATION_ANALYSIS approach again as the default.
• Update the type annotations in Pynguin's code to the simplified, future versions
  (e.g. instead of Dict[str, Set[int]] we can now write dict[str, set[int]]) and do
  not need any imports from Python's typing module.
• Fix a crash of the seeding when native modules are present. Fixes #20.
• Provide a hint in the documentation that PyCharm 2021.3 now integrates poetry
  support, thus no plugin is required for this (and newer) versions (thanks to
  @labrenz).