Description:
RuntimeError thrown when trying to get help for module hashlib

Additional info:
* package version(s)
* config and/or log files etc.


Steps to reproduce:
1) python3
2) import hashlib
3) help(hashlib)


Result:
[sk@oliver ~]$ python3
Python 3.7.0 (default, Jul 15 2018, 10:44:58)
[GCC 8.1.1 20180531] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import hashlib
>>> help(hashlib)
Traceback (most recent call last):
File "/usr/lib/python3.7/inspect.py", line 1996, in wrap_value
value = eval(s, module_dict)
File "<string>", line 1, in <module>
NameError: name '_blake2b' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "/usr/lib/python3.7/inspect.py", line 1999, in wrap_value
value = eval(s, sys_module_dict)
File "<string>", line 1, in <module>
NameError: name '_blake2b' is not defined

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python3.7/_sitebuiltins.py", line 103, in __call__
return pydoc.help(*args, **kwds)
File "/usr/lib/python3.7/pydoc.py", line 1895, in __call__
self.help(request)
File "/usr/lib/python3.7/pydoc.py", line 1954, in help
else: doc(request, 'Help on %s:', output=self._output)
File "/usr/lib/python3.7/pydoc.py", line 1674, in doc
pager(render_doc(thing, title, forceload))
File "/usr/lib/python3.7/pydoc.py", line 1667, in render_doc
return title % desc + '\n\n' + renderer.document(object, name)
File "/usr/lib/python3.7/pydoc.py", line 385, in document
if inspect.ismodule(object): return self.docmodule(*args)
File "/usr/lib/python3.7/pydoc.py", line 1183, in docmodule
contents.append(self.document(value, key, name))
File "/usr/lib/python3.7/pydoc.py", line 386, in document
if inspect.isclass(object): return self.docclass(*args)
File "/usr/lib/python3.7/pydoc.py", line 1237, in docclass
signature = inspect.signature(object)
File "/usr/lib/python3.7/inspect.py", line 3070, in signature
return Signature.from_callable(obj, follow_wrapped=follow_wrapped)
File "/usr/lib/python3.7/inspect.py", line 2820, in from_callable
follow_wrapper_chains=follow_wrapped)
File "/usr/lib/python3.7/inspect.py", line 2343, in _signature_from_callable
return _signature_fromstr(sigcls, obj, text_sig)
File "/usr/lib/python3.7/inspect.py", line 2067, in _signature_fromstr
p(name, default)
File "/usr/lib/python3.7/inspect.py", line 2037, in p
default_node = RewriteSymbolics().visit(default_node)
File "/usr/lib/python3.7/ast.py", line 262, in visit
return visitor(node)
File "/usr/lib/python3.7/inspect.py", line 2024, in visit_Attribute
return wrap_value(value)
File "/usr/lib/python3.7/inspect.py", line 2001, in wrap_value
raise RuntimeError()
RuntimeError