docs: Make pkg_resources warning more visible (#3530)

Author: abravalheri

docs/pkg_resources.rst

@@ -10,12 +10,13 @@ eggs, support for merging packages that have separately-distributed modules or
 subpackages, and APIs for managing Python's current "working set" of active
 packages.
 
-Use of ``pkg_resources`` is discouraged in favor of
-`importlib.resources <https://docs.python.org/3/library/importlib.html#module-importlib.resources>`_,
-`importlib.metadata <https://docs.python.org/3/library/importlib.metadata.html>`_,
-and their backports (:pypi:`importlib_resources`,
-:pypi:`importlib_metadata`).
-Please consider using those libraries instead of pkg_resources.
+.. attention::
+   Use of ``pkg_resources`` is discouraged in favor of
+   `importlib.resources <https://docs.python.org/3/library/importlib.html#module-importlib.resources>`_,
+   `importlib.metadata <https://docs.python.org/3/library/importlib.metadata.html>`_,
+   and their backports (:pypi:`importlib_resources`,
+   :pypi:`importlib_metadata`).
+   Please consider using those libraries instead of pkg_resources.
 
 
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