Set TestData.__test__ to False to avoid pytest discovery

As the class name starts with "Test", pytest interprets it as a test
case, but it is not one. Further, due to its `__init__` method, pytest
emits the following warning:

    .../pip/tests/lib/__init__.py:141: PytestCollectionWarning: cannot
    collect test class 'TestData' because it has a __init__ constructor

For additional information on pytest discovery, see:
https://docs.pytest.org/en/latest/example/pythoncollection.html

Author: jdufresne

news/a9b36e28-beec-4a37-97e1-6dc02ff1eb2d.trivial.rst

@@ -150,6 +150,8 @@ class TestData:
     data into a directory and operating on the copied data.
     """
 
+    __test__ = False
+
     def __init__(self, root, source=None):
         self.source = source or DATA_DIR
         self.root = Path(root).resolve()