Bug: unsupported operand type(s) on Python 3.9

[hemebond]

System info

• OS:

  Linux excession 5.14.0-2-amd64 #1 SMP Debian 5.14.9-2 (2021-10-03) x86_64 GNU/Linux

• Version:

  $ python
  Python 3.9.8 (main, Nov  7 2021, 15:47:09) 
  [GCC 11.2.0] on linux

Describe the bug

When using Python 3.9 importing gimpformats results in a TypeError:

>>> from gimpformats.gimpXcfDocument import GimpDocument
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../gimpformats/gimpXcfDocument.py", line 23, in <module>
    from . import utils
  File ".../gimpformats/utils.py", line 6, in <module>
    def fileOpen(fileName: BytesIO | str) -> tuple[str, bytes]:
TypeError: unsupported operand type(s) for |: 'type' and 'type'

Using a pipe for writing union types is only supported from Python 3.10 (PEP604)