With patch for the zlib module (issue20193) which uses the "unspecified" default value:

$ ./python -m pydoc zlib
Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 1997, in wrap_value
    value = eval(s, module_dict)
  File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 2000, in wrap_value
    value = eval(s, sys_module_dict)
  File "<string>", line 1, in <module>
NameError: name 'unspecified' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 189, in _run_module_as_main
    "__main__", mod_spec)
  File "/home/serhiy/py/cpython/Lib/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 2593, in <module>
    cli()
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 2558, in cli
    help.help(arg)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1840, in help
    elif request: doc(request, 'Help on %s:', output=self._output)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1578, in doc
    pager(render_doc(thing, title, forceload))
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1571, in render_doc
    return title % desc + '\n\n' + renderer.document(object, name)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 356, in document
    if inspect.ismodule(object): return self.docmodule(*args)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1142, in docmodule
    contents.append(self.document(value, key, name))
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 358, in document
    if inspect.isroutine(object): return self.docroutine(*args)
  File "/home/serhiy/py/cpython/Lib/pydoc.py", line 1323, in docroutine
    signature = inspect.signature(object)
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 1468, in signature
    return Signature.from_builtin(obj)
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 2052, in from_builtin
    p(name, default)
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 2038, in p
    default_node = RewriteSymbolics().visit(default_node)
  File "/home/serhiy/py/cpython/Lib/ast.py", line 245, in visit
    return visitor(node)
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 2030, in visit_Name
    return wrap_value(node.id)
  File "/home/serhiy/py/cpython/Lib/inspect.py", line 2002, in wrap_value
    raise RuntimeError()
RuntimeError