Fix test warnings and improve lint configuration

[aphedges]

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• Add unit tests and integration tests where applicable.
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  it to the list under tests/test_definitions.py.
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  you should probably add or change a test in tests/test_integration.py.
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---

I ran the tests locally when I noticed some pytest warnings:

.tox/py36-tests/lib/python3.6/site-packages/_pytest/config/__init__.py:1233
  /Users/ahedges/projects/personal/pydocstyle/.tox/py36-tests/lib/python3.6/site-packages/_pytest/config/__init__.py:1233: PytestConfigWarning: Unknown config option: pep8ignore
  
    self._warn_or_fail_if_strict(f"Unknown config option: {key}\n")

src/tests/test_integration.py:124
  /Users/ahedges/projects/personal/pydocstyle/src/tests/test_integration.py:124: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @pytest.yield_fixture(scope="module")

src/tests/test_integration.py:141
  /Users/ahedges/projects/personal/pydocstyle/src/tests/test_integration.py:141: PytestDeprecationWarning: @pytest.yield_fixture is deprecated.
  Use @pytest.fixture instead; they are the same.
    @pytest.yield_fixture(scope="function", params=['ini', 'toml'])

I also saw a Sphinx warning: WARNING: html_static_path entry '/Users/ahedges/projects/personal/pydocstyle/docs/_static' does not exist.

I decided to fix these warnings.

I can add release notes if you want, but I don't think they are needed for this.

Changes

• Fix pytest deprecation warnings by replacing @pytest.yield_fixture with @pytest.fixture
• Fix pytest unused configuration option warning by removing pep8ignore
  • It was used by pytest-pep8, which was removed from testing in 0a78a1d.
• Fix Sphinx warning by commenting out a line similar to nearby options
  • There hasn't been a _static directory since the the line was first added in e8fd68a.
• Move mypy config from tox.ini to pyproject.toml
  • While this isn't strictly necessary, tox.ini is not one of the places mypy looks for configuration by default, so running mypy by itself was more annoying. The pinned version of mypy supports configuration in pyproject.toml, so it made sense to move it in with the Black and isort configuration.

[Mr-Pepe]

Looks good to me