Add a changelog for TerminalWriter changes

changelog/7135.breaking.rst

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+Pytest now uses its own ``TerminalWriter`` class instead of using the one from the ``py`` library.
+Plugins generally access this class through ``TerminalReporter.writer``, ``TerminalReporter.write()``
+(and similar methods), or ``_pytest.config.create_terminal_writer()``.
+
+The following breaking changes were made:
+
+- Output (``write()`` method and others) no longer flush implicitly; the flushing behavior
+  of the underlying file is respected. To flush explicitly (for example, if you
+  want output to be shown before an end-of-line is printed), use ``write(flush=True)`` or
+  ``terminal_writer.flush()``.
+- Explicit Windows console support was removed, delegated to the colorama library.
+- Support for writing ``bytes`` was removed.
+- The ``reline`` method and ``chars_on_current_line`` property were removed.
+- The ``stringio`` and ``encoding`` arguments was removed.
+- Support for passing a callable instead of a file was removed.