Some technical background.

== Unicode ==

According to the Unicode Standard Annex #9, a character with
bidirectional class B is a "Paragraph Separator". And “Because a
Paragraph Separator breaks lines, there will be at most one per line,
at the end of that line.”

As a consequence, there's 3 reasons to identify a character as a
linebreak:
 - General Category Zl "Line Separator"
 - General Category Zp "Paragraph Separator"
 - Bidirectional Class B "Paragraph Separator"

There's 8 linebreaks in the current Unicode Database (5.2):
------------------------------------------------------------------------
000A    LF  LINE FEED                   Cc  B
000D    CR  CARRIAGE RETURN             Cc  B
001C    FS  INFORMATION SEPARATOR FOUR  Cc  B (UCD 3.1 FILE SEPARATOR)
001D    GS  INFORMATION SEPARATOR THREE Cc  B (UCD 3.1 GROUP SEPARATOR)
001E    RS  INFORMATION SEPARATOR TWO   Cc  B (UCD 3.1 RECORD SEPARATOR)
0085    NEL NEXT LINE                   Cc  B (C1 Control Code)
2028    LS  LINE SEPARATOR              Zl  WS  (Unicode)
2029    PS  PARAGRAPH SEPARATOR         Zp  B   (Unicode)
------------------------------------------------------------------------


== ASCII ==

The Standard ASCII control codes (C0) are in the range 00-1F.
It limits the list to LF, CR, FS, GS, RS.
Regarding the last three, they are not considered as linebreaks:
“The separators (File, Group, Record, and Unit: FS, GS, RS and US) were made to
structure data, usually on a tape, in order to simulate punched cards. End of
medium (EM) warns that the tape (or whatever) is ending. While many systems use
CR/LF and TAB for structuring data, it is possible to encounter the separator
control characters in data that needs to be structured. The separator control
characters are not overloaded; there is no general use of them except to
separate data into structured groupings. Their numeric values are contiguous
with the space character, which can be considered a member of the group, as a
word separator.”
(Ref: http://en.wikipedia.org/wiki/Control_character#Data_structuring)

In conclusion, it may be better to keep things unchanged.
We may add some words to the documentation for str.splitlines() and bytes.splitlines() to explain what is considered a line break character.

References:
 - The Unicode Character Database (UCD): http://www.unicode.org/ucd/
 - UCD Property Values: http://unicode.org/reports/tr44/#Property_Values
 - The Bidirectional Algorithm: http://www.unicode.org/reports/tr9/
 - C0 and C1 Control Codes:
     http://en.wikipedia.org/wiki/C0_and_C1_control_codes