Bug

• OS: ubuntu 18.04
• Python version import sys; print(sys.version): 3.7.5 (default, Nov 7 2019, 10:50:52)
• Pydantic version import pydantic; print(pydantic.VERSION): 1.0b1 or 1.2, but works fine with 0.32

import os
from pydantic import BaseSettings


class MyBaseSettings(BaseSettings):
    var: int = 10


class MySubSettings(MyBaseSettings):
    class Config:
        env_prefix = 'PREFIX_'


os.environ['PREFIX_VAR'] = '2'
k = MySubSettings()
print(k.var)  # 10, but should be 2

We expect that k.var will be equal to 2, but it has default value 10.
It's reproduced since 1.0b1.