MAINT: Remove outdated version checks.

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.
MAINT: Use np.errstate context manager.

Now that Python < 2.6 is no longer supported we can use the errstate
context manager in places where constructs like
```
    old = seterr(invalid='ignore')
    try:
        blah
    finally:
       seterr(**old)
```
were used.
Merge pull request #3516 from charris/remove-outdated-version-checks

MAINT: Remove outdated version checks.
Merge pull request #3518 from charris/use-errstate-context-manager

MAINT: Use np.errstate context manager.
MAINT: Fix Python version dependent test.

The buffer test in numpy/core/tests/test_api.py was apparently
testing for incorrect behavior that was corrected in Python 2.7.5.
This PR changes the test to check for the correct results and makes
running it depend on Python version.

Closes #3512.
Merge pull request #3522 from charris/fix-test_api

MAINT: Fix Python version dependent test.