added yaml loader for person class

.pylintrc

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+[MESSAGES CONTROL]
+
+disable=bad-continuation,
+        missing-docstring

README.md

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+```sh
+sudo apt-get install python3-yaml
+```

conftest.py

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+import os
+import pathlib
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def working_dir():
+    os.chdir(pathlib.Path(__file__).parent.joinpath("tests"))

family_tree_yaml/yaml.py

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+import pathlib
+import typing
+
+import yaml
+from family_tree_yaml import Person
+
+
+class _YamlLoader(yaml.SafeLoader):
+
+    # pylint: disable=too-many-ancestors
+
+    def __init__(self, stream):
+        super().__init__(stream)
+        self.add_constructor("!person", self._construct_person)
+
+    @staticmethod
+    def _construct_person(
+        loader: "_YamlLoader", node: yaml.nodes.MappingNode
+    ) -> Person:
+        (person_attrs,) = loader.construct_yaml_map(node)
+        person = Person(person_attrs["identifier"])
+        if "name" in person_attrs:
+            person.name = person_attrs["name"]
+        if "birth_date" in person_attrs:
+            person.birth_date = person_attrs["birth_date"]
+        return person
+
+
+def read(yaml_path: typing.Union[str, pathlib.Path]) -> Person:
+    """
+    >>> read('persons/max-mustermann.yml')
+    Person(max-mustermann, Max Mustermann, *1976-02-01)
+    >>> read('persons/erika-mustermann.yml')
+    Person(erika-mustermann, Erika Mustermann, *1957-08-12)
+    """
+    if isinstance(yaml_path, str):
+        return read(pathlib.Path(yaml_path))
+    with yaml_path.open("r") as yaml_file:
+        return yaml.load(yaml_file, Loader=_YamlLoader)

tests/persons/erika-mustermann.yml

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+!person
+identifier: erika-mustermann
+name: Erika Mustermann
+birth_date: 1957-08-12

tests/persons/max-mustermann.yml

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+!person
+identifier: max-mustermann
+name: Max Mustermann
+birth_date: 1976-02-01