anki: fix startup

Related: #68314

This fixes startup of anki, which currently shows this in a dialog:

Error during startup:
Traceback (most recent call last):
  File "/nix/store/0h395dwc6b80n5xg93p86ywaz6kpz6ck-anki-2.1.15/lib/python3.7/site-packages/aqt/main.py", line 46, in __init__
    self.setupAddons()
  File "/nix/store/0h395dwc6b80n5xg93p86ywaz6kpz6ck-anki-2.1.15/lib/python3.7/site-packages/aqt/main.py", line 657, in setupAddons
    import aqt.addons
  File "/nix/store/0h395dwc6b80n5xg93p86ywaz6kpz6ck-anki-2.1.15/lib/python3.7/site-packages/aqt/addons.py", line 9, in <module>
    import markdown
  File "/nix/store/knq8798kl0xzzr7ii4bchskg1c8mq6pj-python3.7-Markdown-3.1.1/lib/python3.7/site-packages/markdown/__init__.py", line 25, in <module>
    from .core import Markdown, markdown, markdownFromFile
  File "/nix/store/knq8798kl0xzzr7ii4bchskg1c8mq6pj-python3.7-Markdown-3.1.1/lib/python3.7/site-packages/markdown/core.py", line 29, in <module>
    import pkg_resources
ModuleNotFoundError: No module named 'pkg_resources'