Simpler, fully configured from pyproject.toml, and faster.

It has reached its end of life.

```
Running Sphinx v7.4.6

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/home/freitafr/dev/django-auth-ldap/venv/lib/python3.12/site-packages/sphinx/config.py", line 529, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/home/freitafr/dev/django-auth-ldap/docs/conf.py", line 17, in <module>
    from pkg_resources import DistributionNotFound, get_distribution
ModuleNotFoundError: No module named 'pkg_resources'

make: *** [Makefile:20: html] Error 2
```

Otherwise, the system version is used, which issues a warning, is not
under control and cannot use the pip cache.

Be easier on the network and slightly faster.

…ror signals

* LDAPError exceptions are now propagated from the
  `LDAPSearch.execute()` method, which is used internally in many code
  paths. It's now possible to distinguish errors from empty results.
* In particular `NestedMemberDNGroupType` no longer returns partial
  results when faced with errors, but also propagate the results.
* When `MIRROR_GROUPS` or `MIRROR_GROUPS_EXCEPT` is enabled, then an
  error during group mirroring will fail authentication. Previously it
  could mirror a partial set of groups or remove all groups.
* The `ldap_error` Django signal is now sent for more situations --
  previously it only reported errors from authentication, but nothing
  else.