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/connections.pyi

@@ -0,0 +1,23 @@
+from typing import Any
+
+def prevent_socket_inheritance(sock) -> None: ...
+
+class _ThreadsafeSelector:
+    def __init__(self) -> None: ...
+    def __len__(self): ...
+    @property
+    def connections(self) -> None: ...
+    def register(self, fileobj, events, data: Any | None = ...): ...
+    def unregister(self, fileobj): ...
+    def select(self, timeout: Any | None = ...): ...
+    def close(self) -> None: ...
+
+class ConnectionManager:
+    server: Any
+    def __init__(self, server) -> None: ...
+    def put(self, conn) -> None: ...
+    def stop(self) -> None: ...
+    def run(self, expiration_interval) -> None: ...
+    def close(self) -> None: ...
+    @property
+    def can_add_keepalive_connection(self): ...