Previously, we type the model fields with explicit type annotations
manually with the approximate types. This was because the lack of types
for Django.
django-stubs provides more specific types for all these fields that
incompatible with our previous approximate annotations. So now we can
remove the inline type annotations and rely on the types defined in the
stubs. This allows mypy to infer the types of the model fields for us.
Signed-off-by: Zixuan James Li <p359101898@gmail.com>