Remove `TYPE_CHECKING` from `codecov`

Towards #487 

The code:

```python
from __future__ import annotations
from typing import TYPE_CHECKING
if TYPE_CHECKING:
    from deepinv.xxx import yyy
    ...
```

... is important for static typing (#536) without circular import problems, but it is an extra three lines per file which don't need to be tested. Therefore they should be removed from codecov.