Because Numpy 1.8.0 will no longer supports Python versions < 2.6 we
no longer need to check for that and can also remove the code that is
specific to those earlier versions.

To make this a bit safer, the toplevel setup.py file now contains a
check of the Python version number and raises an error when run by an
unsupported version.

Developers usually only need the type of a return value
followed by a brief description. However, in some cases
providing a name for a return value can make the documentation
clearer. This enhancement changes the format of the Returns
section such that the type is required, and the name is
optional:

    Returns
    -------
    int
        Description of anonymous integer return value.
    x : str
        Description of string return value named `x`.

With this change, if a colon is not present, then the entire
line is interpreted as the return type. In all other cases,
the Returns section is interpreted according to the current
rules.

Consistent with the current format, if a colon is present, then
the text to the left of the colon is interpreted as the name;
and the text to the right of the colon is interpreted as the
type. This makes the proposed change backwards compatible with
existing documentation.

The unicode fixer strips the u from u'hi' and converts the unicode type
to str. The first won't work for Python 2 and instead we replace the u
prefix with the sixu function borrowed from the six compatibility
package. That function calls the unicode constructor with the
'unicode_escape' encoder so that the many tests using escaped unicode
characters like u'\u0900' will be handled correctly. That makes the
sixu function a bit different from the asunicode function currently in
numpy.compat and also provides a target that can be converted back to
the u prefix when support for Python 3.2 is dropped. Python 3.3
reintroduced the u prefix for compatibility.

The unicode fixer also replaces 'unicode' with 'str' as 'unicode' is no
longer a builtin in Python 3. For code compatibility, 'unicode' is
defined either as 'str' or 'unicode' in numpy.compat so that checks like

    if isinstance(x, unicode):
        ...

will work properly for all python versions.

Closes #3089.

The next builtin has been available since Python 2.6 and allows
`it.next()` to be replaced by `next(it)`. In Python 3 the `next` method
is gone entirely, replaced entirely by the `__next__` method. The next
fixer changes all the `it.next()` calls to the new form and renames the
`next` methods to `__next__`. In order to keep  Numpy code backwards
compatible with Python 2, a `next` method was readded to all the Numpy
iterators after the fixer was run so they all contain both methods. The
presence of the appropriate method could have been made version
dependent, but that looked unduly complicated.

Closes #3072.