Unconditional importlib_metadata dependency required for Python 3.8

Describe the bug
After upgrading khal from 0.11.3 to 0.11.4, it throws ModuleNotFoundError: No module named 'importlib_metadata' on every invocation.

Turns out the dependency is only declared for the test extra, but it's actually required on normal use already (though plugins.py → _compat.py).

Requires-Dist: importlib-metadata; python_version <= "3.9" and extra == "test"

If applicable: Stack Trace

$ khal
Traceback (most recent call last):
  File "/usr/local/bin/khal", line 5, in <module>
    from khal.cli import main_khal
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/cli.py", line 33, in <module>
    from . import controllers, plugins
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/controllers.py", line 35, in <module>
    from khal import __productname__, __version__, calendar_display, parse_datetime
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/calendar_display.py", line 29, in <module>
    from .khalendar import CalendarCollection
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/khalendar/__init__.py", line 22, in <module>
    from .khalendar import CalendarCollection  # noqa: F401  # type: ignore
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/khalendar/khalendar.py", line 38, in <module>
    from .event import Event
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/khalendar/event.py", line 41, in <module>
    from ..plugins import FORMATTERS
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/plugins.py", line 3, in <module>
    from khal._compat import importlib_metadata
  File "/opt/pipx/venvs/khal/lib/python3.8/site-packages/khal/_compat.py", line 8, in <module>
    import importlib_metadata
ModuleNotFoundError: No module named 'importlib_metadata'

To Reproduce
Invoke khal.

Expected behavior
No errors.

OS, version, khal version and how you installed it:

• The output of khal --version: khal, version 0.11.4
• Installation method: pipx
• python version: Python 3.8.10
• OS: Ubuntu 20.04.6 LTS

Workaround
Install the dependency manually: pipx inject --global khal importlib-metadata

Start with khal/plugins.py and khal/_compat.py to confirm the runtime import path and reproduce the Python 3.8 failure by invoking khal without the extra dependency. Then inspect the project’s dependency declaration and ensure importlib_metadata is installed for normal use; done means khal starts successfully on Python 3.8 without manual injection.