Fix callable instance variable support (take 2)

[ilevkivskyi]

Fixes #708
Fixes #5485

This builds on the original proposal by @wyfo, but handles three important issues/edge cases:

• This PR fixes serialization of is_inferred so that the distinction works correctly in incremental mode (I added a test)
• Dunder operator methods are always considered class variables (this is a relatively common pattern and matches Python semantics; there is an existing tests that previously needed ClassVar[...])
• If we detect a Too few arguments error for a variable with callable type we give a note suggesting to try ClassVar[...]

I also add a short doc paragraph on this.

cc @JukkaL

[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/execution/execute.py:216: error: Unused "type: ignore" comment
+ src/graphql/execution/execute.py:217: error: Unused "type: ignore" comment
+ src/graphql/execution/execute.py:218: error: Unused "type: ignore" comment
+ src/graphql/execution/execute.py:796: error: Unused "type: ignore" comment

pydantic (https://github.com/samuelcolvin/pydantic)
+ pydantic/validators.py:515: error: Unused "type: ignore" comment

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/local.py:316: error: Unused "type: ignore" comment

sphinx (https://github.com/sphinx-doc/sphinx)
+ sphinx/domains/std.py: note: In member "handle_signature" of class "GenericObject":
+ sphinx/domains/std.py:49:20: error: Too few arguments  [call-arg]
+ sphinx/domains/std.py:49:20: note: "parse_node" is considered instance variable, to make it class variable use ClassVar[...]
+ sphinx/domains/std.py:49:36: error: Argument 1 has incompatible type "BuildEnvironment"; expected "GenericObject"  [arg-type]
+ sphinx/domains/std.py:49:46: error: Argument 2 has incompatible type "str"; expected "BuildEnvironment"  [arg-type]
+ sphinx/domains/std.py:49:51: error: Argument 3 has incompatible type "desc_signature"; expected "str"  [arg-type]

urllib3 (https://github.com/urllib3/urllib3)
+ src/urllib3/connection.py:237: error: Too few arguments  [call-arg]
+ src/urllib3/connection.py:237: note: "_tunnel" is considered instance variable, to make it class variable use ClassVar[...]

jinja (https://github.com/pallets/jinja)
+ src/jinja2/environment.py:1256: error: Unused "type: ignore" comment
+ src/jinja2/environment.py:1352: error: Unused "type: ignore" comment
+ src/jinja2/environment.py:1535: error: Unused "type: ignore" comment
+ src/jinja2/environment.py:1567: error: Unused "type: ignore" comment

bokeh (https://github.com/bokeh/bokeh)
+ bokeh/protocol/receiver.py: note: In member "consume" of class "Receiver":
+ bokeh/protocol/receiver.py:137:9: error: Too few arguments  [call-arg]
+ bokeh/protocol/receiver.py:137:9: note: "_current_consumer" is considered instance variable, to make it class variable use ClassVar[...]
+ bokeh/protocol/receiver.py:137:32: error: Argument 1 has incompatible type "Union[str, bytes]"; expected "Receiver"  [arg-type]

discord.py (https://github.com/Rapptz/discord.py)
- discord/gateway.py:380: error: Cannot assign to a method
- discord/member.py:429: error: Attribute function "create_dm" with type "Callable[[], Awaitable[DMChannel]]" does not accept self argument
- discord/client.py:315: error: Cannot assign to a method
- discord/client.py:316: error: Cannot assign to a method
+ discord/ext/commands/core.py:1958: error: Argument 1 has incompatible type "Context[BotT]"; expected "ContextT"
- discord/ext/commands/core.py:1948: error: Invalid self argument "Check[ContextT]" to attribute function "predicate" with type "Callable[[ContextT_co], Coroutine[Any, Any, bool]]"
- discord/ext/commands/core.py:1958: error: Too many arguments
- discord/ext/commands/core.py:1965: error: Argument 1 to "CheckAnyFailure" has incompatible type "List[Callable[[], Coroutine[Any, Any, bool]]]"; expected "List[Callable[[Context[<nothing>]], bool]]"
+ discord/ext/commands/core.py:1965: error: Argument 1 to "CheckAnyFailure" has incompatible type "List[Callable[[ContextT], Coroutine[Any, Any, bool]]]"; expected "List[Callable[[Context[Any]], bool]]"

[ilevkivskyi]

mypy_primer still looks very good. I checked few new errors, they all look expected (and got useful notes pointing at solution).

[ilevkivskyi]

Since this re-applies previously merged PR, I am going to merge this soon unless there are objections.