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/pyopenssl.pyi

@@ -0,0 +1,30 @@
+from . import Adapter
+from ..makefile import StreamReader, StreamWriter
+from OpenSSL import SSL
+from typing import Any
+
+ssl_conn_type: SSL.Connection
+
+class SSLFileobjectMixin:
+    ssl_timeout: int
+    ssl_retry: float
+    def recv(self, size): ...
+    def readline(self, size: int = ...): ...
+    def sendall(self, *args, **kwargs): ...
+    def send(self, *args, **kwargs): ...
+
+class SSLFileobjectStreamReader(SSLFileobjectMixin, StreamReader): ...  # type:ignore
+class SSLFileobjectStreamWriter(SSLFileobjectMixin, StreamWriter): ...  # type:ignore
+
+class SSLConnectionProxyMeta:
+    def __new__(mcl, name, bases, nmspc): ...
+
+class SSLConnection():
+    def __init__(self, *args) -> None: ...
+
+class pyOpenSSLAdapter(Adapter):
+    def __init__(self, certificate, private_key, certificate_chain: Any | None = ..., ciphers: Any | None = ...) -> None: ...
+    def bind(self, sock): ...
+    def wrap(self, sock): ...
+    def get_environ(self): ...
+    def makefile(self, sock, mode: str = ..., bufsize: int = ...): ...