bpo-28249: fix lineno location for empty DocTest instances (GH-30498)

Co-authored-by: Kumar Aditya <59607654+kumaraditya303@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/doctest_lineno.py

@@ -0,0 +1,50 @@
+# This module is used in `test_doctest`.
+# It must not have a docstring.
+
+def func_with_docstring():
+    """Some unrelated info."""
+
+
+def func_without_docstring():
+    pass
+
+
+def func_with_doctest():
+    """
+    This function really contains a test case.
+
+    >>> func_with_doctest.__name__
+    'func_with_doctest'
+    """
+    return 3
+
+
+class ClassWithDocstring:
+    """Some unrelated class information."""
+
+
+class ClassWithoutDocstring:
+    pass
+
+
+class ClassWithDoctest:
+    """This class really has a test case in it.
+
+    >>> ClassWithDoctest.__name__
+    'ClassWithDoctest'
+    """
+
+
+class MethodWrapper:
+    def method_with_docstring(self):
+        """Method with a docstring."""
+
+    def method_without_docstring(self):
+        pass
+
+    def method_with_doctest(self):
+        """
+        This has a doctest!
+        >>> MethodWrapper.method_with_doctest.__name__
+        'method_with_doctest'
+        """