Typed-argparse

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- Added collision detection for sub parser with same name (including aliases).
- Fixed handling of sub parser aliases.
- Fixed handling of optional positional arguments.
- Added support for metavar.
- Fixed type correctness of arguments with field annotations.
- Added basic support for default handling in manual constructor call.
- Added support for union type expressions.

- Make constructor of `TypedArgs` adhere to type semantics of dataclass transform.
- Drop work-around and backports for Python 3.6 and 3.7

- Fix regression introduced by `typing_extensions` version 4.6.0+.

- Support for `formatter_class` in `Parser`.

- Fix issue when parser is used under `from __future__ import annotations` context.