Cythonized data classes allow omission of required arguments.

[kollmats]

Describe the bug

When instantiating a data class, it usually detects if a required argument is missing and raises an error. In Cython, nothing happens until you try to access the missing field and an AttributeError is thrown:

Traceback (most recent call last):
File "", line 1, in
File "main.py", line 24, in init main
AttributeError: 'Bb' object has no attribute 'a1'

To Reproduce

from dataclasses import dataclass

@dataclass                                                                                           
class Aa:                                                                                            
    a1: str                                                                                          
    a2: str = '100'                                                                                  
    a3: str = None                                                                                   
                                                                                                      
class Bb(Aa):                                                                                        
    b1: str = '123'                                                                                  
    b2: str = None                                                                                   
                                                                                                      
bb = Bb()
print(bb.a1)

Expected behavior
Without Cython, the above yields the following traceback when calling the constructor:

Traceback (most recent call last):
File "main.py", line 23, in
bb = Bb()
TypeError: init() missing 1 required positional argument: 'a1'

Environment (please complete the following information):

OS: Ubuntu 
Version: 18.0.4

Additional context
Python 3.6.9, Cython 0.29.21, dataclasses 0.7 (backport)

[scoder]

Duplicate of #2552.