Docs: Remove note in installation guide regarding Python requirement

This admonition is not that useful since it still refers the user to an
external website and the target information is not immediately obvious
either. Besides, if the installed Python version is not supported, this
will automatically be reported by the package manager.

docs/source/installation/guide_quick.rst

@@ -10,11 +10,6 @@ First, install the ``aiida-core`` Python package:
 
     pip install aiida-core
 
-.. attention::
-
-    AiiDA requires a recent version of Python.
-    Please refer to the `Python Package Index <https://pypi.org/project/aiida-core/>`_ for the minimum required version.
-
 Next, set up a profile where all data is stored:
 
 .. code-block:: console