[now-python] Fix missing sys modules for relative module import (#4024)
[now-python] Fix missing sys modules for relative module import (#4024)

Without this fix, Python app which uses [Pydantic](https://pydantic-docs.helpmanual.io/usage/postponed_annotations/) for data validation will crash with log like this:

```py
  File "pydantic/main.py", line 175, in pydantic.main.ModelMetaclass.__new__
    annotations = resolve_annotations(namespace.get('__annotations__', {}), namespace.get('__module__', None))
  File "pydantic/typing.py", line 136, in pydantic.typing.resolve_annotations
    annotations = resolve_annotations(namespace.get('__annotations__', {}), namespace.get('__module__', None))
  File "pydantic/typing.py", line 136, in pydantic.typing.resolve_annotations
    base_globals: Optional[Dict[str, Any]] = sys.modules[module_name].__dict__
KeyError: 'api.main'
```

The added code also follow Python official documentation: https://docs.python.org/3.6/library/importlib.html#importing-a-source-file-directly

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