test using Sphinx<3.1 due to issues w/ annotations
test using Sphinx<3.1 due to issues w/ annotations

With Sphinx 3.1 the following

```python
from nutils.points import Points
from typing import Union, overload
import numpy

class PointsSequence:

  ...

  @overload
  def __getitem__(self, index: int) -> Points:
    ...
  @overload
  def __getitem__(self, index: Union[slice, numpy.ndarray]) -> 'PointsSequence':
    ...
  def __getitem__(self, index):
    '''Return ``self[index]``.'''
```

generates the warnings

```
docstring of PointsSequence.__getitem__:: WARNING: py:class reference target not found: Points
docstring of PointsSequence.__getitem__:: WARNING: py:class reference target not found: 'PointsSequence'
```

joostvanzwieten/nutils,wijnandhoitinga/nutils