refactor: Show deprecation warning on root context manager

This context manager can delete the entire instance of ModIO when using
`with rpi:` after creating an instance of ModIO by
`rpi = revpimodio.RevPiModIO(...)`.

src/revpimodio2/modio.py

@@ -208,6 +208,19 @@ class RevPiModIO(object):
                 self._myfh.close()
 
     def __enter__(self):
+        # todo: Remove this context manager in future
+        warnings.warn(
+            "This context manager is deprecated and will be removed!\n\n"
+            "You should use the context manager of the IO object `with revpi.io:` "
+            "or with a single device `with revpi.device.my_device:`.\n\n"
+            "This deprecated context manager can be reproduced as follows:\n"
+            "```"
+            "revpi = revpimodio2.RevPiModIO()"
+            "with revpi.io:"
+            "    ..."
+            "```",
+            DeprecationWarning,
+        )
         if self._context_manager:
             raise RuntimeError("can not use multiple context managers of same instance")
         if self._looprunning: