Only require attrdict3 on Windows

It's only used on Windows, so only require it there.

buildtools/config.py

@@ -27,8 +27,6 @@ from distutils.dep_util  import newer
 
 import distutils.sysconfig
 
-from attrdict import AttrDict
-
 runSilently = False
 
 #----------------------------------------------------------------------
@@ -992,6 +990,8 @@ def getMSVCInfo(PYTHON, arch, set_env=False):
     if MSVCinfo is not None:
         return MSVCinfo
 
+    from attrdict import AttrDict
+
     # Note that it starts with a monkey-patch in setuptools.msvc to
     # workaround this issue: pypa/setuptools#1902
     cmd = \

requirements/devel.txt

@@ -21,4 +21,4 @@ Jinja2==2.10
 markupsafe==1.1.1
 doc2dash==2.3.0
 beautifulsoup4
-attrdict3
+attrdict3 ; sys_platform == 'win32'