replace np.str with str

Summary:
`np.str` is removed from numpy 1.20.0. It was an alias to builtin `str` and it's safe to do the replacement.

The whole changes is mechanical, generated using the following onliner:
```
fbgr -sl 'np\.str\b' | xargs perl -pi -e 's,\bnp\.str\b,str,g'
```

Differential Revision: D46586144

fbshipit-source-id: 4dd09345409294e1ba6db48f2201a074a2ae5c05

detectron2/data/datasets/pascal_voc.py

@@ -32,7 +32,7 @@ def load_voc_instances(dirname: str, split: str, class_names: Union[List[str], T
  class_names: list or tuple of class names
  """
  with PathManager.open(os.path.join(dirname, "ImageSets", "Main", split + ".txt")) as f:
- fileids = np.loadtxt(f, dtype=np.str)
+ fileids = np.loadtxt(f, dtype=str)
 
  # Needs to read many small annotation files. Makes sense at local
  annotation_dirname = PathManager.get_local_path(os.path.join(dirname, "Annotations/"))