Stockholm

* Python 3.10 added to test matrix and trove classifiers to officially claim support.

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* Major updates to improve type hints and intellisense within editors.
* Reworked the currency classes to utilize the metaclass in a better way.
* Additional updates to ease development working with Protocol Buffers and monetary amounts (mostly related to better type hint annotations which gives a better developer experience).
* Updates to the readme with additional examples.
* Dropped support for Python 3.6.

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* Python 3.9 supported.
* Minor type annotation fixes.

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* Fixes an issue that caused a monetary amount without currency to get a `"None"` string instead of an empty string as value to `currency_code` when creating a protobuf message using the `.as_protobuf()` method.

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* Added support for conversion to and from Protocol Buffers using the new `Money.from_protobuf` or `money.as_protobuf` functions. By default using the [`google.type.Money`](https://github.com/googleapis/googleapis/blob/master/google/type/money.proto) protobuf definition.
* Instantiation of monetary object by passing JSON data and the possibility to get a JSON string based on a monetary amount object.
* Added documentation. Mostly regarding the use and examples about Protocol Buffers, but also a whole new section about the properties that are available on the `stockholm.Money` object.

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* Updated versions for test suite (`flake8`, `pytest`, `mypy`, among others) and then corrected a newly found type hinting issue tracked down by the updated versions.
* Added target-versions of the supported Python versions for `black` to not accidentally break any backwards compatibility.
* Fixes default decimal digit formatting of the Ugandan shilling (*currency code **UGX***). [Thanks tritas]

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