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/ssl/__init__.pyi

@@ -0,0 +1,19 @@
+from abc import abstractmethod
+from typing import Any
+
+class Adapter():
+    certificate: Any
+    private_key: Any
+    certificate_chain: Any
+    ciphers: Any
+    context: Any
+    @abstractmethod
+    def __init__(self, certificate, private_key, certificate_chain: Any | None = ..., ciphers: Any | None = ...): ...
+    @abstractmethod
+    def bind(self, sock): ...
+    @abstractmethod
+    def wrap(self, sock): ...
+    @abstractmethod
+    def get_environ(self): ...
+    @abstractmethod
+    def makefile(self, sock, mode: str = ..., bufsize: int = ...): ...