incompatible typevar-likes with same name

e.g.

```python
type A[T: str] = T | bool
type B[T: int] = T | bool
```

currently raises

```
NotImplementedError: conflicting typevar definitions: 'T'
```

instead, it should result in something like

```python
from typing import TypeAlias, TypeVar

_T1 = TypeVar("_T1", bound=str)
_T2 = TypeVar("_T2", bound=int)

A: TypeAlias = _T1 | bool
B: TypeAlias = _T2 | bool
```

