adapt to new import.metadata.entry_points API

[bertsky]

This fixes a regression which prevents running OCR-D Browser on Python < 3.10 and >= 3.8.

The reason is an ill-conceived change in the importlib.metadata API: Starting with importlib_metadata>=5.0 (recently released), you can no longer directly use the old API, which directly accesses the dict from entry_points(). Alas, the new API is not available in native importlib.metadata for Python 3.8 and Python 3.9.

There is a backport shim specifically for that problem, but I really resent that additional complication.

I therefore just switched to importlib_metadata >= 3.6 and the new API throughout.

[bertsky]

@hnesk I have not tested this on Python 3.10 yet. Your CI also does not include a setup for that version, but more importantly, somehow the PR does not even trigger the CI workflow… EDIT it does (was only delayed)

[hnesk]

What a mess, thanks for fixing @bertsky !