Fixes how ** affects inference in {} expressions, refs #11691

[sobolevn]

This is a quite common problem, I've seen multiple issue reports about this.

So, the problem is: when a person uses {**kwargs}, it uses the same logic we have for .update() call. But, this is not right. .update() indeed needs matching types.
But, unpacking into a new dict should be allowed with any values.

To achive that I've decided to use join for key and value types. This is how it works:

You may ask: why not Union? dict[Union[type, float], str] looks strange to me. Also, some other similar places use join. But, I am open to discuss this.

I will add extra tests a bit later, I am currently too tired 😞

Closes #11691
Refs #11151

[sobolevn]

This is a readability change only. Before and after:

[sobolevn]

I think that we also need to handle: NoReturn and TypedDict types here. Any others?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

graphql-core (https://github.com/graphql-python/graphql-core)
+ src/graphql/utilities/extend_schema.py:200: error: Unused "type: ignore" comment
+ src/graphql/utilities/extend_schema.py:245: error: Unused "type: ignore" comment
+ src/graphql/utilities/extend_schema.py:353: error: Unused "type: ignore" comment
+ src/graphql/utilities/extend_schema.py:362: error: Unused "type: ignore" comment

pandera (https://github.com/pandera-dev/pandera)
+ pandera/schemas.py:324: error: Incompatible return value type (got "Dict[Any, object]", expected "Dict[str, str]")  [return-value]

rclip (https://github.com/yurijmikhalevich/rclip)
- rclip/db.py:70: error: Argument 1 to "update" of "dict" has incompatible type "NewImage"; expected "Mapping[Any, None]"

ibis (https://github.com/ibis-project/ibis)
- ibis/backends/base/sql/registry/main.py:361: error: Argument 1 to "update" of "dict" has incompatible type "Dict[AnnotableMeta, Any]"; expected "Mapping[Type[Node], Any]"
- ibis/backends/impala/compiler.py:25: error: Argument 1 to "update" of "dict" has incompatible type "Dict[AnnotableMeta, Any]"; expected "Mapping[Type[Node], Any]"
+ ibis/backends/clickhouse/compiler.py:69: error: Incompatible types in assignment (expression has type "Dict[Type[Node], Any]", base class "ExprTranslator" defined the type as "Dict[AnnotableMeta, Any]")

black (https://github.com/psf/black)
+ src/blib2to3/pgen2/tokenize.py:581:31: error: Incompatible types in assignment (expression has type "Optional[Pattern[str]]", variable has type "Pattern[str]")
+ src/blib2to3/pgen2/tokenize.py:603:29: error: Incompatible types in assignment (expression has type "Optional[Pattern[str]]", variable has type "Pattern[str]")

core (https://github.com/home-assistant/core)
+ homeassistant/components/sentry/__init__.py:207: error: Unused "type: ignore" comment