allow path-like objects to be cwd on windows (#389)

#157 added the test, but it's currently (correctly) broken on windows.
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index dffcda3..23e9bd3 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -988,7 +988,7 @@
                                          int(not close_fds),
                                          creationflags,
                                          env,
-                                         cwd,
+                                         os.fspath(cwd),
                                          startupinfo)
             finally:
                 # Child is launched. Close the parent's copy of those pipe