I’ve decided I would prefer deprecating the Latin-1 encoding, rather than adding more encoding support for iterables and text files. If not deprecating it altogether, at least prefer just ASCII encoding (like Python 2). The urlopen() function already rejects text str, and in Issue 26045 people often want or expect UTF-8.

Here is a summary I made of the different data types handled for the body object, from highest priority to lowest priority.

             HTTPConnection   HTTPConn. default                    
body         _send_output()   Content-Length     urlopen()         
===========  ===============  =================  ==================
None         No body          0 or unset #23539  C-L unset         
Has read()   read() #1065257                     * #5038           
- Text file  encode() #5314                                        
str          encode() r56128  len()              TypeError #11082  
Byte seq.    sendall()        len()              C-L: nbytes       
Bytes-like   * #27340         *                  C-L: nbytes #3243 
Iterable     iter() #3243                        C-L required #3243
Sized                         len()† #23350                        
fstat()                       st_size #1065257                     
Otherwise    TypeError        Unset #1065257     C-L optional†     

* Possible bugs
† Degenerate cases not worth supporting IMO
C-L = Content-Length header field, set by default or required to be specified in various cases
Byte seq. = Sequence of bytes, including “bytes” type, bytearray, array("B"), etc
Bytes-like = Any C-contiguous buffer, including zero- and multi-dimensional arrays, and with items other than bytes
Iterable = Iterable of bytes or bytes-like objects (depending on Issue 27340 about SSL)