TypeScript - Cannot access static members of model with .schema() call

Issue Description

When you call .schema() method of model then call static member typescript says that member doesn't exist.

What are you doing?

class TestModelWithStaticMethod extends Model {
  static someStaticFn() {
    return 'hello!';
  }
};
TestModelWithStaticMethod.init({ hello: { type: DataTypes.BOOLEAN, allowNull: false } }, { sequelize });
const testModelWithStaticMethod = TestModelWithStaticMethod.schema('some_schema');
testModelWithStaticMethod.someStaticFn();

What do you expect to happen?

Ability to use static members of model without warnings.

What is actually happening?

> tsc -b types/tsconfig.json && tsc -b types/test/tsconfig.json

types/test/model.ts:86:27 - error TS2339: Property 'someStaticFn' does not exist on type '(new () => TestModelWithStaticMethod) & typeof Model'.

86 testModelWithStaticMethod.someStaticFn();

Environment

• Sequelize version: 5.x and 6.x
• Node.js version: v13.2.0
• Operating System: macOS 10.15.3
• If TypeScript related: TypeScript version: 3.7.2

Issue Template Checklist

How does this problem relate to dialects?

• I think this problem happens regardless of the dialect.
• I think this problem happens only for the following dialect(s):
• I don't know, I was using PUT-YOUR-DIALECT-HERE, with connector library version XXX and database version XXX

Would you be willing to resolve this issue by submitting a Pull Request?

• Yes, I have the time and I know how to start.
• Yes, I have the time but I don't know how to start, I would need guidance.
• No, I don't have the time, although I believe I could do it if I had the time...
• No, I don't have the time and I wouldn't even know how to start.

Start with the failing example in types/test/model.ts and the TypeScript build commands shown in the error. Trace the type returned by Model.schema(), then update the typing so subclass static members remain available and confirm both listed TypeScript builds pass.