Remove dependency on modbus for stiebel_eltron (#136482)

* Remove dependency on modbus for stiebel_eltron

The modbus integration changed its setup, so it is
not possible anymore to have an empty hub.

* Add config flow

* Update pystiebeleltron to v0.1.0

* Fix

* Fix

* Add test for non existing modbus hub

* Fix tests

* Add more tests

* Add missing translation string

* Add test for import failure

* Fix issues from review comments

* Fix issues from review comments

* Mock stiebel eltron client instead of setup_entry

* Update homeassistant/components/stiebel_eltron/__init__.py

* Update homeassistant/components/stiebel_eltron/__init__.py

---------

Co-authored-by: Joostlek <joostlek@outlook.com>

CODEOWNERS

@@ -1474,7 +1474,8 @@ build.json @home-assistant/supervisor
 /tests/components/steam_online/ @tkdrob
 /homeassistant/components/steamist/ @bdraco
 /tests/components/steamist/ @bdraco
-/homeassistant/components/stiebel_eltron/ @fucm
+/homeassistant/components/stiebel_eltron/ @fucm @ThyMYthOS
+/tests/components/stiebel_eltron/ @fucm @ThyMYthOS
 /homeassistant/components/stookwijzer/ @fwestenberg
 /tests/components/stookwijzer/ @fwestenberg
 /homeassistant/components/stream/ @hunterjm @uvjustin @allenporter