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/builtin.pyi

@@ -0,0 +1,18 @@
+from typing import Any
+from . import Adapter
+
+generic_socket_error: OSError
+DEFAULT_BUFFER_SIZE: int
+
+class BuiltinSSLAdapter(Adapter):
+    CERT_KEY_TO_ENV: Any
+    CERT_KEY_TO_LDAP_CODE: Any
+    def __init__(self, certificate, private_key, certificate_chain: Any | None = ..., ciphers: Any | None = ...) -> None: ...
+    @property
+    def context(self): ...
+    @context.setter
+    def context(self, context) -> None: ...
+    def bind(self, sock): ...
+    def wrap(self, sock): ...
+    def get_environ(self): ...
+    def makefile(self, sock, mode: str = ..., bufsize: int = ...): ...