To avoid byte/character mixture, getarg('c') must only accept a byte 
string of 1 byte and getarg('C') only an unicode string of 1 
character.

Impacted methods:
- datetime.datetime.isoformat(sep), array.array(type, data): don't 
accept byte anymore
- msvcrt.putch(char), msvcrt.ungetch(char), <mmap 
object>.write_byte(char): don't accept unicode anymore

I tried runtests.sh: only the 3 bytes.{center,ljust,rjust} tests have 
to be changed, all other tests are ok.

Related issues: #3446, #5391.