Remove too restrictive __all__ definitions
Remove too restrictive `__all__` from `libcommon/simple_cache`:
- it contained only one attribute: `DoesNotExist`
- Python considers as private all the module attributes which are not defined in `__all__`

```
from libcommon.simple_cache import upsert_response

Warning: Accessing a protected member of a class or a module:
'upsert_response' is not declared in __all__
```
Remove too restrictive __all__ definitions: Remove too restrictive `__all__` from `libcommon/simple_cache`:
- it contained only one attribute: `DoesNotExist`
- Python considers as private all the module attributes which are not defined in `__all__`

```
from libcommon.simple_cache import upsert_response

Warning: Accessing a protected member of a class or a module:
'upsert_response' is not declared in __all__
```