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

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

homeassistant/generated/config_flows.py

@@ -603,6 +603,7 @@ FLOWS = {
         "starlink",
         "steam_online",
         "steamist",
+        "stiebel_eltron",
         "stookwijzer",
         "streamlabswater",
         "subaru",