Fix --open-files test with Python 3.3 and newer

Currently, the pytest plugin for open files uses a function that has been deprecated since [this commit](https://hg.python.org/cpython/rev/b81ddaf0db47) in Python 3.3.

Specifically, running `python setup.py test --open-files` gives: (shortening the long lines)
```
astropy/tests/pytest_plugins.py:431: 
 _

    def _get_open_file_list():
        fsencoding = sys.getfilesystemencoding()
    
        sproc = subprocess.Popen(
            ['lsof -F0 -n -p {0}'.format(os.getpid())],
            shell=True, stdout=subprocess.PIPE)
        output = sproc.communicate()[0].strip()
        files = []
        for line in output.split(b'\n'):
            columns = line.split(b'\0')
            mapping = {}
            for column in columns:
                if len(column) >= 2:
                    mapping[column[0:1]] = column[1:]
    
            if (mapping.get(b'f') and
                mapping.get(b'a', b' ') != b' ' and
                    mapping.get(b't') == b'REG'):
                # Ignore extension modules -- they may be imported by a
                # test but are never again closed by the runtime.  That's
                # ok.
>               for suffix, mode, filetype in imp.get_suffixes():
                    if mapping[b'n'].decode(fsencoding).endswith(suffix):

astropy/tests/pytest_plugins.py:418: 
 _

    def get_suffixes():
        warnings.warn('imp.get_suffixes() is deprecated; use the constants '
                      'defined on importlib.machinery instead',
>                     DeprecationWarning, 2)
E       DeprecationWarning: imp.get_suffixes() is deprecated; use the constants defined on importlib.machinery instead

/usr/lib64/python3.3/imp.py:58: DeprecationWarning
= 96 skipped, 11 xfailed, 5561 error in 200.14 seconds =
```
While `python2.7 setup.py test --open-files` only results in:
```
= 1 failed, 8090 passed, 149 skipped, 43 xfailed, 3 xpassed in 478.00 seconds =
```
(the one failure looks like a numerical issue in `TestFittable1DModels.test_deriv_1D`)


After this PR, the numbers for python 3 go down to:
```
= 1 failed, 5482 passed, 140 skipped, 42 xfailed, 3 xpassed in 292.71 seconds =
```
and is unchanged for python2.

To prevent this from happening again, I also enabled Travis to check for opened files with the newest python2 and python3 versions.
+1 to @embray's suggestion