Nattrs

- Updates pyproject.toml to new poetry version.

- Fixes path to hexagon badge when viewing outside of GitHub.

- Adds `nested_delattr()` for deleting attributes / dict members
in nested data structures.
- Adds `nested_updattr()` for updating dicts and `class.__dict__` of
nested attributes with a `dict`.
- Adds `regex` argument to `nested_*attr()` functions for using
regular expression to index attribute names / dict keys.
- Adds `getter_default` argument to `nested_mutattr()` to allow
better handling of missing attributes.
- Adds `Ignore()` class to use as `default` value in `nested_getattr()`
and `getter_default` in `nested_mutattr()`.
- Removes mutability requirements from type hints when they are
unnecessary (only the sub object/Mapping needs to by mutable).
- Adds hexagon bagde.

- Adds `__version__` variable to package.
- Changes type hints from `dict` to `Mapping` and `MutableMapping`
since the functions should work for those in general.

- Handles `zipp` vulnerability.
- Now requires `Python==3.8` or newer.

- Reformats README