gh-136914: Fix support of cached functions and properties in DocTest's lineno computation (GH-136930)

Previously, DocTest's lineno of functions and methods decorated with
functools.cache(), functools.lru_cache() and functools.cached_property()
was not properly returned (None was returned) because the
computation relied on inspect.isfunction() which does not consider the
decorated result as a function.

We now use the more generic inspect.isroutine(), as elsewhere
in doctest's logic.

Also, added a special case for functools.cached_property().

Lib/doctest.py

@@ -94,6 +94,7 @@ def _test():
 
 import __future__
 import difflib
+import functools
 import inspect
 import linecache
 import os
@@ -1141,7 +1142,9 @@ def _find_lineno(self, obj, source_lines):
         if inspect.ismethod(obj): obj = obj.__func__
         if isinstance(obj, property):
             obj = obj.fget
-        if inspect.isfunction(obj) and getattr(obj, '__doc__', None):
+        if isinstance(obj, functools.cached_property):
+            obj = obj.func
+        if inspect.isroutine(obj) and getattr(obj, '__doc__', None):
             # We don't use `docstring` var here, because `obj` can be changed.
             obj = inspect.unwrap(obj)
             try: