Add session.warn to output warnings (#482)

* Add `session.warn` to output warnings

Show warnings during the session.

* Update test_sessions.py

Add a test for "session.warn"
(It is pretty similar to "test_log", but it has changed the logging level to be tested).

* Update test_sessions.py

Run black to fix linting errors.

* Update test_sessions.py

Remove an additional whiespace.

nox/sessions.py

@@ -516,6 +516,10 @@ def log(self, *args: Any, **kwargs: Any) -> None:
         """Outputs a log during the session."""
         logger.info(*args, **kwargs)
 
+    def warn(self, *args: Any, **kwargs: Any) -> None:
+        """Outputs a warning during the session."""
+        logger.warning(*args, **kwargs)
+
     def error(self, *args: Any) -> "_typing.NoReturn":
         """Immediately aborts the session and optionally logs an error."""
         raise _SessionQuit(*args)

tests/test_sessions.py

@@ -601,6 +601,14 @@ def test_log(self, caplog):
 
         assert "meep" in caplog.text
 
+    def test_warn(self, caplog):
+        caplog.set_level(logging.WARNING)
+        session, _ = self.make_session_and_runner()
+
+        session.warn("meep")
+
+        assert "meep" in caplog.text
+
     def test_error(self, caplog):
         caplog.set_level(logging.ERROR)
         session, _ = self.make_session_and_runner()