Fix async auth client

[adamjmcgrath]

Changes

Asyncifying authentication classes doesn't work because the asyncify method was assuming the wrong signature.

References

fixes #498

Testing

AsyncGetToken = asyncify(GetToken)

get_token = AsyncGetToken(domain, client_id)

response = await self.get_token.login_async(
    username=username,
    password=password,
    realm="Username-Password-Authentication",
    scope="openid profile email",
)

Checklist

• I have read the Auth0 general contribution guidelines
• I have read the Auth0 Code of Conduct
• All existing and new tests complete without errors