[0.5.8] Fix typing (#15)

* `to_dict()` output type is a `dict`
* Target Python versions are 3.9-3.13
* Type checking with latest version of pyright

[0.5.7] fix treatment of default nested instance (#13)

Here is an example of default nested instance:
```
class Nested(NamedTuple):
    nested: int

class Config(NamedTuple):
    item1: Nested
    item2: Nested = Nested(2)
```

Here `item1` caused no problem while `item2` was treated improperly.

[0.5.6] wrap `open` with context (#10)

* wrap `open` with context

* readme

Set up continuous integration actions (#2)

* Create CI.yml

* Update CI.yml

* Update CI.yml

* Update SETUP.py

* add typing_extensions as a dependency

* Add pytest-mock dependency

* Update CI.yml

* Update CI.yml

Upload pytest results

* Update README.md

* Add CD action

* update README