Nest config flow (#14921)

* Move nest to dir based component

* Add config flow for Nest

* Load Nest platforms via config entry

* Add tests for Nest config flow

* Import existing access tokens as config entries

* Lint

* Update coverage

* Update translation

* Fix tests

* Address strings

* Use python-nest token resolution

* Lint

* Do not do I/O inside constructor

* Lint

* Update test requirements

homeassistant/data_entry_flow.py

@@ -132,7 +132,8 @@ class FlowHandler:
     VERSION = 1
 
     @callback
-    def async_show_form(self, *, step_id, data_schema=None, errors=None):
+    def async_show_form(self, *, step_id, data_schema=None, errors=None,
+                        description_placeholders=None):
         """Return the definition of a form to gather user input."""
         return {
             'type': RESULT_TYPE_FORM,
@@ -141,6 +142,7 @@ class FlowHandler:
             'step_id': step_id,
             'data_schema': data_schema,
             'errors': errors,
+            'description_placeholders': description_placeholders,
         }
 
     @callback