pycompat: export a handle on concurrent.futures

On Python 3, we use the built-in version in the standard library. Else
we use our vendored backport.

Differential Revision: https://phab.mercurial-scm.org/D3266

mercurial/pycompat.py

@@ -26,7 +26,10 b' if not ispy3:'
     import Queue as _queue
     import SocketServer as socketserver
     import xmlrpclib
+
+    from .thirdparty.concurrent import futures
 else:
+    import concurrent.futures as futures
     import http.cookiejar as cookielib
     import http.client as httplib
     import pickle