Consolidate API types

[giampaolo]

From https://github.com/giampaolo/psutil/blob/master/psutil/tests/test_unicode.py:

On Python 3 the returned paths are supposed to match 100% (and this
is tested).
Not on Python 2 though, where we assume correct unicode path handling
is broken. In fact it is broken for most os.* functions, see:
http://bugs.python.org/issue18695
There really is no way for psutil to handle unicode correctly on
Python 2 unless we make such APIs return a unicode type in certain
circumstances.
I'd rather have unicode support broken on Python 2 than having APIs
returning variable str/unicode types, see:
https://github.com/giampaolo/psutil/issues/655#issuecomment-136131180

As such we also test that all APIs on Python 2 always return str and
never unicode (in test_contracts.py).

Basically we assume unicode support is broken on Python 2 and want to make sure unicode is never returned. I added some tests about this (see: 35ebdfb) already.
The only place were we may want to return unicode is WindowsService.description() so this serves as a reminder for that (needs investigation).

Also, I'm adding tests to make sure the same types (int, float, whatever..., see d67154d, 5aa3af3 and test_contracts.py) are returned across across platforms and I already bumped in a couple of errors on OSX but I expect there will be more.

[giampaolo]

OSX : https://travis-ci.org/giampaolo/psutil/jobs/227283028

[giampaolo]

Windows: https://ci.appveyor.com/project/giampaolo/psutil/build/1152/job/s9d2526hkj6ge7g4

======================================================================
FAIL: psutil.tests.test_unicode.TestNonFSAPIS.test_proc_environ
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\projects\psutil\psutil\tests\test_unicode.py", line 271, in test_proc_environ
    self.assertIsInstance(k, str)
AssertionError: u'APPVEYOR_REPO_COMMIT_AUTHOR' is not an instance of <type 'str'>
----------------------------------------------------------------------

[giampaolo]

Windows:

=====================================================================
FAIL: psutil.tests.test_contracts.TestFetchAllProcesses.test_fetch_all
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\users\user\desktop\psutil\psutil\tests\test_contracts.py", line 338, in test_fetch_all
    meth(ret, p)
  File "c:\users\user\desktop\psutil\psutil\tests\test_contracts.py", line 458, in cpu_times
    self.assertIsInstance(n, float)
AssertionError: 0 is not an instance of <type 'float'>

======================================================================
FAIL: psutil.tests.test_contracts.TestSystem.test_net_if_stats
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\users\user\desktop\psutil\psutil\tests\test_contracts.py", line 223, in test_net_if_stats
    self.assertIsInstance(ifname, str)
AssertionError: u'Ethernet 3' is not an instance of <type 'str'>

======================================================================
FAIL: psutil.tests.test_system.TestSystemAPIs.test_net_if_stats
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\users\user\desktop\psutil\psutil\tests\test_system.py", line 642, in test_net_if_stats
    self.assertIsInstance(name, str)
AssertionError: u'Ethernet 3' is not an instance of <type 'str'>

======================================================================
FAIL: psutil.tests.test_unicode.TestNonFSAPIS.test_proc_environ
----------------------------------------------------------------------
Traceback (most recent call last):
  File "c:\users\user\desktop\psutil\psutil\tests\test_unicode.py", line 270, in test_proc_environ
    self.assertIsInstance(k, str)
AssertionError: u'TMP' is not an instance of <type 'str'>

[giampaolo]

I will handle this in #1040 as a bigger task. Closing this out as duplicate.

[giampaolo]

Actually I will handle the str type issue in the other ticket and the rest here.

[giampaolo]

OK, this should be fixed by now.