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/_code/code.py

@@ -38,6 +38,7 @@ from _pytest._io import TerminalWriter
 from _pytest._io.saferepr import safeformat
 from _pytest._io.saferepr import saferepr
 from _pytest.compat import ATTRS_EQ_FIELD
+from _pytest.compat import final
 from _pytest.compat import get_real_func
 from _pytest.compat import overload
 from _pytest.compat import TYPE_CHECKING
@@ -414,6 +415,7 @@ co_equal = compile(
 _E = TypeVar("_E", bound=BaseException, covariant=True)
 
 
+@final
 @attr.s(repr=False)
 class ExceptionInfo(Generic[_E]):
     """Wraps sys.exc_info() objects and offers help for navigating the traceback."""