rust-cpython: move borrow_mut() to PySharedRefCell PySharedRefCell() will host almost all py_shared public functions. This change is the first step. borrow_mut() can be safely implemented since PySharedRefCell knows its inner object is managed by its own py_shared_state.

rust-cpython: move py_shared_state to PySharedRefCell object The goal of this series is to encapsulate more "py_shared" thingy and reduce the size of the macro, which is hard to debug. Since py_shared_state manages the borrowing state of the object owned by PySharedRefCell, this change makes more sense. If a PyObject has more than one data to be leaked into Python world, each PySharedState should incref the parent PyObject, and keep track of the corresponding borrowing state.

py3: decode username and password before SMTP login smtplib.SMTP.login() requires str on Python 3. For 'password', we only need to decode when value comes from config as getpass() returns the correct type already.

py3: use socket.makefile() instead of dropped smtplib.SSLFakeFile The latter (undocumented internal) class got removed in Python 3.3. Use socket.makefile() as suggested: https://docs.python.org/3/whatsnew/3.3.html#porting-python-code

py3: call SMTP.docmd() with an str Otherwise, this always returns (502, '5.5.2 Error: command not recognized').

py3: call SMTP.has_extn() with an str Passing a bytes on Python 3 always returns False, thus starttls is not properly detected.