Changing return type of `__iter__` to `Iterator`.

[johanols]

The previous return type of Iterable is incorrect as can be seen in this example: https://docs.python.org/3/library/typing.html#nominal-vs-structural-subtyping

Also this documentation mentions that __iter__ returns the iterator object itself: https://docs.python.org/3/library/stdtypes.html#iterator.__iter__

The incorrect Iterable type causes numerous of error when analyzed by a static type checker such as mypy. The switch to Iterator solves these issues.

Checklist

• [x] Added a docs/changes.rst entry

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (a6d0bbe) 0.00% compared to head (104ad9f) 0.00%. Report is 2 commits behind head on main.

Files	Patch %	Lines
minimalkv/decorator.py	0.00%	4 Missing :warning:
minimalkv/_key_value_store.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #99 +/- ## ===================================== Coverage 0.00% 0.00% ===================================== Files 32 32 Lines 1993 1993 ===================================== Misses 1993 1993 ```

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