Minor python changes (#832)

* Minor python changes

Use dataclasses instead of attrs. The former is part of the standard lib
while the latter may need to be installed separately.

Also added python3 to the list of prerequisites.

config/nom_build.py

@@ -16,7 +16,7 @@
 """
 
 import argparse
-import attr
+import dataclasses
 import io
 import os
 import shutil
@@ -25,7 +25,7 @@ import sys
 from typing import List, Union
 
 
-@attr.s(auto_attribs=True)
+@dataclasses.dataclass
 class Property:
     name : str = ''
     desc : str = ''
@@ -39,7 +39,7 @@ class Property:
                 raise ValidationError('value of {self.name} must be "true" or '
                                       '"false".')
 
-@attr.s(auto_attribs=True)
+@dataclasses.dataclass
 class GradleFlag:
     flags : Union[str, List[str]]
     desc : str