Fix broken installation on Windows in virtualenv

Fixes #4142. Python `site.py` from any virtualenv does not contain many functions added in python 2.7 (see: pypa/virtualenv#355), which results in method `site.getsitepackages` being absent.

This commit replaces getsitepackages call with equivalent of `distutils.sysconfig.get_python_lib` which is supported in virtualenv.

Author: AleksanderGondek

mypy/build.py

@@ -13,12 +13,12 @@
 import binascii
 import collections
 import contextlib
+from distutils.sysconfig import get_python_lib
 import gc
 import hashlib
 import json
 import os.path
 import re
-import site
 import sys
 import time
 from os.path import dirname, basename
@@ -229,7 +229,7 @@ def default_data_dir(bin_dir: Optional[str]) -> str:
     """
     if not bin_dir:
         if os.name == 'nt':
-            prefixes = [os.path.join(sys.prefix, 'Lib'), os.path.join(site.getuserbase(), 'lib')]
+            prefixes = [os.path.join(sys.prefix, 'Lib'), os.path.join(get_python_lib(), 'lib')]
             for parent in prefixes:
                     data_dir = os.path.join(parent, 'mypy')
                     if os.path.exists(data_dir):