sys: Fix sys.maxsize and add sys.maxunicode

Current implementation of sys.maxsize is unsigned type
but AFAIK the type of sys.maxsize should be identical with
Py_ssize_t which is known as signed type.

sys.maxunicode is hardcoded from cpython repository
https://github.com/python/cpython/blob/master/Python/sysmodule.c#L2654

Author: corona10

tests/snippets/sysmod.py

@@ -20,3 +20,4 @@
 assert type(sys.flags).__name__ == "flags"
 assert type(sys.flags.optimize) is int
 assert sys.flags[3] == sys.flags.optimize
+assert sys.maxunicode == 1114111

vm/src/sysmodule.rs

@@ -256,7 +256,8 @@ settrace() -- set the global debug tracing function
       "getfilesystemencoding" => ctx.new_rustfunc(sys_getfilesystemencoding),
       "getfilesystemencodeerrors" => ctx.new_rustfunc(sys_getfilesystemencodeerrors),
       "intern" => ctx.new_rustfunc(sys_intern),
-      "maxsize" => ctx.new_int(std::usize::MAX),
+      "maxunicode" => ctx.new_int(0x0010_FFFF),
+      "maxsize" => ctx.new_int(std::isize::MAX),
       "path" => path,
       "ps1" => ctx.new_str(">>>>> ".to_string()),
       "ps2" => ctx.new_str("..... ".to_string()),