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/makefile.pyi

@@ -0,0 +1,32 @@
+import io
+
+SOCK_WRITE_BLOCKSIZE: int
+
+class BufferedWriter(io.BufferedWriter):
+    def write(self, b): ...
+
+class MakeFile_PY2:
+    bytes_read: int
+    bytes_written: int
+    def __init__(self, *args, **kwargs) -> None: ...
+    def write(self, data) -> None: ...
+    def send(self, data): ...
+    def flush(self) -> None: ...
+    def recv(self, size): ...
+    class FauxSocket: ...
+    def read(self, size: int = ...): ...
+    def readline(self, size: int = ...): ...
+    def has_data(self): ...
+
+class StreamReader(io.BufferedReader):
+    bytes_read: int
+    def __init__(self, sock, mode: str = ..., bufsize=...) -> None: ...
+    def read(self, *args, **kwargs): ...
+    def has_data(self): ...
+
+class StreamWriter(BufferedWriter):
+    bytes_written: int
+    def __init__(self, sock, mode: str = ..., bufsize=...) -> None: ...
+    def write(self, val, *args, **kwargs): ...
+
+def MakeFile(sock, mode: str = ..., bufsize=...): ...