Patch the _curses module to improve Unicode support:

 - add an encoding attribute to a window (only visible in C): read the locale encoding
 - encode a character and a character string to the window encoding if the ncursesw library is NOT used
 - addch(), addstr(), addnstr(), insstr() and insnstr() use the wide character functions if the ncursesw library is used
 - PyCurses_ConvertToChtype() checks for integer overflow and rejects values outside [0; 255]

The check on the ncursesw library availability is done in setup.py because the library linked to _curses depends on the readline library (see issues #7384 and #9408).

I don't know if wide character functions can be available in curses or ncurses library.

Details:

 - locale encoding: use GetConsoleOutputCP() on Windows, nl_langinfo(CODESET) if available, or "utf-8"
 - don't encode a character to the window encoding if its code is in [0; 127] (use the Unicode point code): all encoding are compatible with ASCII... except some encodings like JIS X 0201. In JIS, 0x5C is decoded to the yen sign (U+00A5) instead of a backslash (U+005C).
 - if an encoded character is longer than 1 byte, raise a OverflowError. For example, U+00E9 (é) encoded to UTF-8 gives b'\xC3\xA9' (two bytes).
 - copy the encoding when creating a subwindow.
 - use a global variable, screen_encoding, in PyCurses_UnCtrl() and PyCurses_UngetCh()

It's not possible to specify an encoding.

GetConsoleOutputCP() is maybe not the right code on Windows if a text application doesn't run in a Windows console (e.g. if it uses its own terminal emulator). GetOEMCP() is maybe a better choice, or a function should be added to specify the encoding used by the _curses module (override the "locale encoding").

If a function is added to specify the encoding, I think that it is better to add a global function instead of adding an argument to functions creating a new window object (initscr(), getwin(), subwin(), derwin(), newpad()).