Python 3.10 Do not list NotRequired in __required_keys__

It suprises me that this went unnotices and tested. NotRequired is a feature of 3.11 but the backport is lacking on <=3.10, as the following example shows

```python
from typing_extensions import TypedDict, NotRequired

class Foo(TypedDict):
     a : NotRequired[int]


print(Foo.__required_keys__)  # should be empty.
# frozenset({'a'})
```