Mark some public and to-be-public classes as @final

This indicates at least for people using type checkers that these
classes are not designed for inheritance and we make no stability
guarantees regarding inheritance of them.

Currently this doesn't show up in the docs. Sphinx does actually support
`@final`, however it only works when imported directly from `typing`,
while we import from `_pytest.compat`.

In the future there might also be a `@sealed` decorator which would
cover some more cases.

src/_pytest/_io/terminalwriter.py

@@ -7,6 +7,7 @@ from typing import Sequence
 from typing import TextIO
 
 from .wcwidth import wcswidth
+from _pytest.compat import final
 
 
 # This code was initially copied from py 1.8.1, file _io/terminalwriter.py.
@@ -36,6 +37,7 @@ def should_do_markup(file: TextIO) -> bool:
     )
 
 
+@final
 class TerminalWriter:
     _esctable = dict(
         black=30,