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/scope.py

@@ -71,7 +71,8 @@ class Scope(Enum):
         from _pytest.outcomes import fail
 
         try:
-            return Scope(scope_name)
+            # Holding this reference is necessary for mypy at the moment.
+            scope = Scope(scope_name)
         except ValueError:
             fail(
                 "{} {}got an unexpected scope value '{}'".format(
@@ -79,6 +80,7 @@ class Scope(Enum):
                 ),
                 pytrace=False,
             )
+        return scope
 
 
 _ALL_SCOPES = list(Scope)