Issue #14687: Optimize str%args

Issue #14687: Optimize str%args

 * formatfloat() uses unicode_fromascii() instead of PyUnicode_DecodeASCII()
   to not have to check characters, we know that it is really ASCII
 * Use PyUnicode_FromOrdinal() instead of _PyUnicode_FromUCS4() to format
   a character: if avoids a call to ucs4lib_find_max_char() to compute
   the maximum character (whereas we already know it, it is just the character
   itself)