`password` is a valid OpenAPIv3 string format, that is used as a UI hint
for frontend clients to mask the input field.
It was already present in the `SchemaFormat` enum, but it was not
handled in `_unmarshal_string` that uses `STRING_FORMAT_CALLABLE_GETTER`
to decide how to unmarshal a string, which would result in an error like
this one:
```
Traceback (most recent call last):
[... snip ...]
File ".venv/lib/python3.7/site-packages/openapi_core/validation/request/validators.py", line 37, in validate
body, body_errors = self._get_body(request, operation)
File ".venv/lib/python3.7/site-packages/openapi_core/validation/request/validators.py", line 82, in _get_body
body = media_type.unmarshal(raw_body, self.custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/media_types/models.py", line 45, in unmarshal
unmarshalled = self.schema.unmarshal(deserialized, custom_formatters=custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 189, in unmarshal
casted = self.cast(value, custom_formatters=custom_formatters, strict=strict)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 179, in cast
return cast_callable(value)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 295, in _unmarshal_object
value, custom_formatters=custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 335, in _unmarshal_properties
prop_value, custom_formatters=custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 189, in unmarshal
casted = self.cast(value, custom_formatters=custom_formatters, strict=strict)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 179, in cast
return cast_callable(value)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 295, in _unmarshal_object
value, custom_formatters=custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 335, in _unmarshal_properties
prop_value, custom_formatters=custom_formatters)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 189, in unmarshal
casted = self.cast(value, custom_formatters=custom_formatters, strict=strict)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 179, in cast
return cast_callable(value)
File ".venv/lib/python3.7/site-packages/openapi_core/schema/schemas/models.py", line 215, in _unmarshal_string
formatstring = self.STRING_FORMAT_CALLABLE_GETTER[schema_format]
KeyError: <SchemaFormat.PASSWORD: 'password'>
```
This is important because it does the correct validation over items that
are restricted in "oneOf", so that it's possible to use schemas that are
superset of one another as items of "oneOf".
Before this change, if a UUID instance got received as value in the
Schema, it was breaking the unmarshal because UUID instances can't be
used as values to instantiate other UUIDs.
