Include new features from Python 3.8 and 3.9

[WeisLeDocto]

As detailed in #63, it is more than time to drop support for Python 3.7 and 3.8. This update allows using the features introduced in Python 3.8 and 3.9, which is what this PR does. The following improvements are brought to the code base:

• Use of the walrus operator :=
• Use of the typing.Literal class for improving documentation
• Replacement of dict.update() calls with |=
• Use of keyword-only and positional-only arguments
• Replacement of typing.Tuple, typing.List, typing.Dict and typing.Type with their associated builtins
• Use of typing.TypeVar to improve some type hints
• Replacement of typing.Iterator, typing.Iterable and typing.Callable with their collections.abc equivalents

closes #63