py: Add stream reading of n unicode chars; unicode support by default.
With unicode enabled, this patch allows reading a fixed number of characters from text-mode streams; eg file.read(5) will read 5 unicode chars, which can made of more than 5 bytes. For an ASCII stream (ie no chars > 127) it only needs to do 1 read. If there are lots of non-ASCII chars in a stream, then it needs multiple reads of the underlying object. Adds a new test for this case. Enables unicode support by default on unix and stmhal ports.