Deprecation of msg= for both pytest.skip() and pytest.fail(). (#8950)

* porting pytest.skip() to use reason=, adding tests

* avoid adding **kwargs, it breaks other functionality, use optional msg= instead

* deprecation of `pytest.fail(msg=...)`

* fix bug with not capturing the returned reason value

* pass reason= in acceptance async tests instead of msg=

* finalising deprecations of `msg` in `pytest.skip()` and `pytest.fail()`

* Update doc/en/deprecations.rst

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

* Update doc/en/deprecations.rst

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

* fix failing test after upstream merge

* adding deprecation to `pytest.exit(msg=...)`

* add docs for pytest.exit deprecations

* finalising deprecation of msg for pytest.skip, pytest.exit and pytest.fail

* hold a reference to the Scope instance to please mypy

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>

testing/test_main.py

@@ -70,7 +70,7 @@ def test_wrap_session_exit_sessionfinish(
         """
         import pytest
         def pytest_sessionfinish():
-            pytest.exit(msg="exit_pytest_sessionfinish", returncode={returncode})
+            pytest.exit(reason="exit_pytest_sessionfinish", returncode={returncode})
     """.format(
             returncode=returncode
         )