Fix type-based code completion for some LSPs

[ikalnytskyi]

It turns out that picobox.Stack() is missing few key type annotations that breaks autocompletion for some LSP servers. For instance, in the following code example:

@dataclasses.dataclass
class Struct:
    value: int

@picobox.pass_("value")
def pack(value: int) -> Struct:
    return Struct(value=value)

with picobox.push(picobox.Box()) as box:
    box.put("value", 42)
    value = pack(42).value

there are two broken autocompletion points:

• .put() is not completed for the box variable
• .value is not completed for the struct type returned by pack()

This patch fixes these scenarios by (1) annotating that picobox.push() returns a context manager, and (2) avoiding using too-much-dynamic expressions in picobox.pass_() which return type cannot be easily inferred.