Support user install on Windows. (#4005)

On Windows, the usual install paths are `sys.prefix/Lib/mypy` for a global
install. For user installs, the path is `site.getuserbase()/lib/mypy`. We
also fall back to the old method of installation due to the data dir being
put in the package directory on an egg install.

This restores the usual data directory resolution on other platforms.
Correctly fixes #3988, and keeps other platforms working.

Author: emmatyping

mypy/build.py

@@ -17,6 +17,7 @@
 import json
 import os.path
 import re
+import site
 import sys
 import time
 from os.path import dirname, basename
@@ -213,6 +214,12 @@ def default_data_dir(bin_dir: Optional[str]) -> str:
       bin_dir: directory containing the mypy script
     """
     if not bin_dir:
+        if os.name == 'nt':
+            prefixes = [os.path.join(sys.prefix, 'Lib'), os.path.join(site.getuserbase(), 'lib')]
+            for parent in prefixes:
+                    data_dir = os.path.join(parent, 'mypy')
+                    if os.path.exists(data_dir):
+                        return data_dir
         mypy_package = os.path.dirname(__file__)
         parent = os.path.dirname(mypy_package)
         if (os.path.basename(parent) == 'site-packages' or
@@ -223,13 +230,10 @@ def default_data_dir(bin_dir: Optional[str]) -> str:
             # or .../blah/lib64/python3.N/dist-packages/mypy/build.py (Gentoo)
             # or .../blah/lib/site-packages/mypy/build.py (Windows)
             # blah may be a virtualenv or /usr/local.  We want .../blah/lib/mypy.
-            # On Windows, if the install is .../python/PythonXY/site-packages, we want
-            # .../python/lib/mypy
             lib = parent
             for i in range(2):
                 lib = os.path.dirname(lib)
-                if os.path.basename(lib) in ('lib', 'lib32', 'lib64') \
-                        or os.path.basename(lib).startswith('python'):
+                if os.path.basename(lib) in ('lib', 'lib32', 'lib64'):
                     return os.path.join(os.path.dirname(lib), 'lib/mypy')
         subdir = os.path.join(parent, 'lib', 'mypy')
         if os.path.isdir(subdir):