No module named 'aiostream' in python39+

Before you submit bug reports: https://vdirsyncer.pimutils.org/en/stable/contributing.html

• vdirsyncer, version 0.19.0
• freebsd port py39-vdirsyncer
• python Python 3.9.16
• FreeBSD 12.4

error: Unknown error occurred: No module named 'aiostream'
error: Use -vdebug to see the full traceback.
debug: File "/usr/local/lib/python3.9/site-packages/vdirsyncer/cli/init.py", line 32, in inner
debug: f(*a, **kw)
debug: File "/usr/local/lib/python3.9/site-packages/vdirsyncer/cli/init.py", line 126, in sync
debug: from .tasks import prepare_pair
debug: File "/usr/local/lib/python3.9/site-packages/vdirsyncer/cli/tasks.py", line 8, in
debug: from .discover import collections_for_pair
debug: File "/usr/local/lib/python3.9/site-packages/vdirsyncer/cli/discover.py", line 8, in
debug: import aiostream

Replace :

import aiostream

With :
if sys.version_info < (3,9):
import aiostream

Fixes the bug, Would you accept PR?

Start in vdirsyncer/cli/discover.py, where the traceback shows aiostream is imported, and review the callers in vdirsyncer/cli/tasks.py and vdirsyncer/cli/init.py. Verify the supported Python versions and dependency packaging before choosing a fix; done means syncing no longer fails with a missing aiostream module on the reported environment.