Release 1.2.1 breaks Python 3.8

[brentyi]

Hello!

It looks like this line:

mediapy/mediapy/__init__.py

Line 191 in c51276b

_Path = typing.Union[str, os.PathLike[str]]

breaks Python 3.8, because os.PathLike[str] isn't subscriptable.

Here's the full error:

>>> import mediapy
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/brentyi/miniconda3/envs/py38/lib/python3.8/site-packages/mediapy/__init__.py", line 191, in <module>
    _Path = typing.Union[str, os.PathLike[str]]
TypeError: 'ABCMeta' object is not subscriptable

cc nerfstudio-project/nerfstudio#3189