Fix deprecation warnings from pytest

[sobolevn]

.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:108
  /Users/sobolev/Documents/github/returns/.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:108: PytestDeprecationWarning: direct construction of YamlTestFile has been deprecated, please use YamlTestFile.from_parent
    return YamlTestFile(path, parent=parent, config=parent.config)

.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89
.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89
.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89
.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89
.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89
  /Users/sobolev/Documents/github/returns/.venv/lib/python3.8/site-packages/pytest_mypy_plugins/collect.py:89: PytestDeprecationWarning: direct construction of YamlTestItem has been deprecated, please use YamlTestItem.from_parent
    yield YamlTestItem(

-- Docs: https://docs.pytest.org/en/latest/warnings.html