Initial python3 changes

Mostly just updating libraries, removing string encoding/decoding,
fixing some edge cases. No new functionality was added in this
commit.

lib/cheroot/workers/threadpool.pyi

@@ -0,0 +1,37 @@
+import threading
+from typing import Any
+
+class TrueyZero:
+    def __add__(self, other): ...
+    def __radd__(self, other): ...
+
+trueyzero: TrueyZero
+
+class WorkerThread(threading.Thread):
+    conn: Any
+    server: Any
+    ready: bool
+    requests_seen: int
+    bytes_read: int
+    bytes_written: int
+    start_time: Any
+    work_time: int
+    stats: Any
+    def __init__(self, server): ...
+    def run(self) -> None: ...
+
+class ThreadPool:
+    server: Any
+    min: Any
+    max: Any
+    get: Any
+    def __init__(self, server, min: int = ..., max: int = ..., accepted_queue_size: int = ..., accepted_queue_timeout: int = ...) -> None: ...
+    def start(self) -> None: ...
+    @property
+    def idle(self): ...
+    def put(self, obj) -> None: ...
+    def grow(self, amount) -> None: ...
+    def shrink(self, amount) -> None: ...
+    def stop(self, timeout: int = ...) -> None: ...
+    @property
+    def qsize(self) -> int: ...