Removed instructions to create a PostgreSQL cluster in GIS docs

Creating a new cluster is neither required not recommendable for
most users. The previous section explains how to create a user
with sufficient permissions to automatically create a database
during tests.
Fixed #23923 -- Promoted Django's deprecation warnings to errors in runtests.py
Fixed #17890 -- Added an extra_context parameter to AdminSite.password_change().
Fixed #23930 -- Added copies of captured_std* managers from CPython's test.support.

StringIO import was adapted for compatibility with Python 2.