Fix docs, pkg_resources not found

[sdarwin]

Building boost docs on Ubuntu 26.04 Sphinx 9.1.0.
pkg_resources is no longer available.
I tested the current pipeline and the modification seems to be compatible.

Error:

None.
Loaded Extensions
# None.
Traceback
      File "/opt/venvboostdocs/lib/python3.14/site-packages/sphinx/config.py", line 611, in eval_config_file
        raise ConfigError(msg % traceback.format_exc()) from exc
    sphinx.errors.ConfigError: There is a programmable error in your configuration file:
   
    Traceback (most recent call last):
      File "/opt/venvboostdocs/lib/python3.14/site-packages/sphinx/config.py", line 596, in eval_config_file
        exec(code, namespace) # NoQA: S102
        ~~~~^^^^^^^^^^^^^^^^^
      File "/opt/boostorg/boost-root/libs/hof/doc/conf.py", line 27, in <module>
        from pkg_resources import parse_version
    ModuleNotFoundError: No module named 'pkg_resources'
The full traceback has been saved in:
/tmp/sphinx-err-64pd5pq1.log