Update types for ThreadPoolExecutor for py3.6 (#1414)

* Update types for ThreadPoolExecutor for py3.6

* Removed Optional from ThreadPoolExecutor thread_name_prefix argument

Author: miggaiowski

stdlib/3/concurrent/futures/thread.pyi

@@ -1,5 +1,10 @@
 from typing import Optional
 from ._base import Executor, Future
+import sys
 
 class ThreadPoolExecutor(Executor):
-    def __init__(self, max_workers: Optional[int] = ...) -> None: ...
+    if sys.version_info >= (3, 6):
+        def __init__(self, max_workers: Optional[int] = ...,
+                     thread_name_prefix: str = ...) -> None: ...
+    else:
+        def __init__(self, max_workers: Optional[int] = ...) -> None: ...