Tab completion Fraction(n,m).denomi<TAB> causes "Exception There are no params defined on this."

Here is how to reproduce the problem:

```
$ ipython
Python 3.7.3 (default, Jun 11 2019, 18:15:26)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.5.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from fractions import Fraction as fr

In [2]: fr(2,3).denomi
```

Now, at this point press `TAB` and then the `In [2]...` line disappears and we get this:

```
Unhandled exception in event loop:
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/buffer.py", line 1648, in new_coroutine
yield From(coroutine(*a, **kw))
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/eventloop/coroutine.py", line 90, in step_next
new_f = coroutine.throw(exc)
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/buffer.py", line 1500, in async_completer
cancel=lambda: not proceed()))
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/eventloop/coroutine.py", line 86, in step_next
new_f = coroutine.send(None)
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/eventloop/async_generator.py", line 114, in consume_async_generator
item = iterator.send(send)
File "/usr/local/lib/python3.7/site-packages/prompt_toolkit/completion/base.py", line 170, in get_completions_async
for item in self.get_completions(document, complete_event):
File "/usr/local/lib/python3.7/site-packages/IPython/terminal/ptutils.py", line 90, in get_completions
yield from self._get_completions(body, offset, cursor_position, self.ipy_completer)
File "/usr/local/lib/python3.7/site-packages/IPython/terminal/ptutils.py", line 100, in _get_completions
for c in completions:
File "/usr/local/lib/python3.7/site-packages/IPython/core/completer.py", line 445, in _deduplicate_completions
completions = list(completions)
File "/usr/local/lib/python3.7/site-packages/IPython/core/completer.py", line 1820, in completions
for c in self._completions(text, offset, _timeout=self.jedi_compute_type_timeout/1000):
File "/usr/local/lib/python3.7/site-packages/IPython/core/completer.py", line 1877, in _completions
signature = _make_signature(jm)
File "/usr/local/lib/python3.7/site-packages/IPython/core/completer.py", line 996, in _make_signature
return '(%s)'% ', '.join([f for f in (_formatparamchildren(p) for p in completion.params) if f])
File "/usr/local/lib/python3.7/site-packages/jedi/cache.py", line 143, in wrapper
result = method(self, *args, **kwargs)
File "/usr/local/lib/python3.7/site-packages/jedi/api/classes.py", line 345, in params
raise AttributeError('There are no params defined on this.')

Exception There are no params defined on this.
Press ENTER to continue...
In [2]: fr(2,3).denomi
```

(The `ENTER` was already pressed, so the `In [2]...` line came back). You can see from the above listing that the version of Python is 3.7.3 and the version of IPython is 7.5.0.