Consolidate config in pyproject.toml (#15548)

There's no real need for us to have separate `setup.cfg` and
`pytest.ini` files these days

pyproject.toml

@@ -79,3 +79,47 @@ extra-standard-library = ["typing_extensions"]
 
 [tool.check-manifest]
 ignore = ["**/.readthedocs.yaml"]
+
+[tool.pytest.ini_options]
+minversion = "6.0.0"
+testpaths = ["mypy/test", "mypyc/test"]
+python_files = 'test*.py'
+
+# Where do the test cases come from?  We provide our own collection
+# logic by implementing `pytest_pycollect_makeitem` in mypy.test.data;
+# the test files import that module, and pytest sees the magic name
+# and invokes it at the relevant moment.  See
+# https://doc.pytest.org/en/latest/how-to/writing_plugins.html#collection-hooks
+
+# Both our plugin and unittest provide their own collection logic,
+# So we can disable the default python collector by giving it empty
+# patterns to search for.
+# Note that unittest requires that no "Test*" classes exist.
+python_classes = []
+python_functions = []
+
+# always run in parallel (requires pytest-xdist, see test-requirements.txt)
+# and enable strict mode: require all markers
+# to be defined and raise on invalid config values
+addopts = "-nauto --strict-markers --strict-config"
+
+# treat xpasses as test failures so they get converted to regular tests as soon as possible
+xfail_strict = true
+
+[tool.coverage.run]
+branch = true
+source = "mypy"
+parallel = true
+
+[tool.coverage.report]
+show_missing = true
+skip_covered = true
+omit = 'mypy/test/*'
+exclude_lines = [
+  '\#\s*pragma: no cover',
+  '^\s*raise AssertionError\b',
+  '^\s*raise NotImplementedError\b',
+  '^\s*return NotImplemented\b',
+  '^\s*raise$',
+  '''^if __name__ == ['"]__main__['"]:$''',
+]