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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)