Add stricter mypy type check flags

[antonagestam]

• Remove unused type: ignore.
• Remove asterisk import and add __all__ with all exported names, to
  pass with the no_explicit_reexport mypy flag.
• Add various mypy config for developer ergonomics (prettier errors,
  printing error codes, warning about unused config, ignores and casts
  etc). Kept warn_return_any = False in dacite.types for now since there
  would have to be a lot of ignores/casts otherwise.

This is a somewhat opinionated PR. The added __all__ makes the public API of the package more explicit and enables consumers of the library to type check with no_explicit_reexport = True in mypy. Adding python_version = 3.6 helps finding incompatibilities, such as trying to import Literal from stdlib typing, with mypy. dacite already passes with most of the stricter settings except for one found unnecessary ignore (warn_unused_ignores) and one added type hint in dacite.dataclasses.create_instance(). The exception is the dacite.types module where mypy isn't very successful at inferring return types, so I set warn_return_any = False for that file.

As I said I consider this opinionated, but do feel free to ask for changes if you're interested in some or all of these changes.