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>

src/_pytest/python.py

@@ -175,7 +175,7 @@ def async_warn_and_skip(nodeid: str) -> None:
     msg += "  - pytest-trio\n"
     msg += "  - pytest-twisted"
     warnings.warn(PytestUnhandledCoroutineWarning(msg.format(nodeid)))
-    skip(msg="async def function and no async plugin installed (see warnings)")
+    skip(reason="async def function and no async plugin installed (see warnings)")
 
 
 @hookimpl(trylast=True)