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/testing.pyi

@@ -0,0 +1,17 @@
+from typing import Any, Iterator, Optional, TypeVar
+
+from .server import HTTPServer
+from .wsgi import Server
+
+T = TypeVar('T', bound=HTTPServer)
+
+EPHEMERAL_PORT: int
+NO_INTERFACE: Optional[str]
+ANY_INTERFACE_IPV4: str
+ANY_INTERFACE_IPV6: str
+config: dict
+
+def cheroot_server(server_factory: T) -> Iterator[T]: ...
+def wsgi_server() -> Iterator[Server]: ...
+def native_server() -> Iterator[HTTPServer]: ...
+def get_server_client(server) -> Any: ...