[red-knot] Implicit instance attributes

[sharkdp]

Summary

Add support for implicitly-defined instance attributes, i.e. support type inference for cases like this:

class C:
    def __init__(self) -> None:
        self.x: int = 1

reveal_type(C().x)  # int

To do:

• Support explicitly declared attribute assignments (self.x: str = "a")
• Support declarations (self.x: str)
• Support non-annotated assignments (self.x = "a")
• Build unions of inferred types, if there are multiple assignments
• Write tests for nested classes
• Write tests for functions nested inside __init__
• Write tests for attributes whose type should be a union from different assignments (in multiple functions).
• Write tests for something like self.x, self.y = …
• Write tests for conditional function definitions

Things intentionally left out of this PR:

• Type inference for self
• Support attributes which are implicitly "declared" via their parameter types (self.x = param)
• Tuple-unpacking attribute assignments: self.x, self.y = …
• Diagnostic for conflicting declared types

Test Plan

Updated and new Markdown tests

[sharkdp]

I think this doesn't work because we don't know what the type of self is.

[sharkdp]

See above: this does not work because we don't know what the type of self is.

[sharkdp]

Probably not something that we're going to address here. This would require narrowing on attribute expressions. Unknown | Literal["value set in class method"] is not wrong.