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/python_api.py

@@ -17,6 +17,7 @@ from typing import TypeVar
 from typing import Union
 
 import _pytest._code
+from _pytest.compat import final
 from _pytest.compat import overload
 from _pytest.compat import STRING_TYPES
 from _pytest.compat import TYPE_CHECKING
@@ -699,6 +700,7 @@ def raises(  # noqa: F811
 raises.Exception = fail.Exception  # type: ignore
 
 
+@final
 class RaisesContext(Generic[_E]):
     def __init__(
         self,