Support edges based on TypeVar constraints/bounds

[jayqi]

typing.TypeVar are used for annotating generics, and they can take constraints or bounds.

It probably makes sense to consider those to be an edge. TypeVar annotations are currently not considered a model. To support this, we'll need to special case TypeVar instances and introspect them to get this information.

Example of current behavior:

from typing import Generic, TypeVar, Optional, List

import erdantic as erd
from pydantic import BaseModel, field_validator
import rich

class DataModel(BaseModel):
    """Payload representation."""
    numbers: List[int]
    people: List[str]

DataT = TypeVar('DataT', DataModel, str)

class Response(BaseModel, Generic[DataT]):
    """HTTP Response representation."""

    data: Optional[DataT]

diagram = erd.create(Response)
rich.print(diagram)
#> EntityRelationshipDiagram(models={'__main__.Response': ModelInfo(...)}, edges={})

Desired:

An edge from Response to DataModel should be detected.