add pipe2() to the os module

Doc/library/os.rst

 :mod:`os` --- Miscellaneous operating system interfaces
 =======================================================
 
 .. module:: os
    :synopsis: Miscellaneous operating system interfaces.
 
 
 This module provides a portable way of using operating system dependent
 functionality.  If you just want to read or write a file see :func:`open`, if
 you want to manipulate paths, see the :mod:`os.path` module, and if you want to
@@ skipping 1001 matching lines @@
 
 
 .. function:: pipe()
 
    Create a pipe.  Return a pair of file descriptors ``(r, w)`` usable for reading
    and writing, respectively.
 
    Availability: Unix, Windows.
 
 
-.. function:: pipe2(flags=0)
+.. function:: pipe2(flags)
 
    Create a pipe with *flags* set atomically.
-   *flags* is optional and can be constructed by ORing together zero or more of
-   these values: :data:`O_NONBLOCK`, :data:`O_CLOEXEC`.
+   *flags* can be constructed by ORing together one or more of these values:
+   :data:`O_NONBLOCK`, :data:`O_CLOEXEC`.
    Return a pair of file descriptors ``(r, w)`` usable for reading and writing,
    respectively.
 
    Availability: some flavors of Unix.
+
+   .. versionadded:: 3.3
 
 
 .. function:: posix_fallocate(fd, offset, len)
 
    Ensures that enough disk space is allocated for the file specified by *fd*
    starting from *offset* and continuing for *len* bytes.
 
    Availability: Unix.
 
    .. versionadded:: 3.3
@@ skipping 1819 matching lines @@
 
 .. function:: urandom(n)
 
    Return a string of *n* random bytes suitable for cryptographic use.
 
    This function returns random bytes from an OS-specific randomness source.  The
    returned data should be unpredictable enough for cryptographic applications,
    though its exact quality depends on the OS implementation.  On a UNIX-like
    system this will query /dev/urandom, and on Windows it will use CryptGenRandom.
    If a randomness source is not found, :exc:`NotImplementedError` will be raised.