CLEAN: cleanup up platform / python version checks

[jreback]

we have in various locations

os.name, platform.system(), sys.platform

for detecting platform. Recently add are these functions
in pandas/compat/__init__.py as is_platform_* which are reasonable to standardize on

however, need to find & fix all the other references to make this nice and clean
this includes code and tests

Ideally also add: skip_if_windows functions in pandas/util/testing.py (and replace the use of the direct comparisons) would be gr8.

futhermore any references to to python 3 checks should use PY3 (in pandas/compat):

./pandas/tseries/tests/test_timeseries.py:
   50 :     if sys.version_info > (3,) and sys.platform == 'win32':
   54 :     if sys.version_info < (3,) or sys.platform != 'win32':

I think we could use an update on the wiki on the same

[jreback]

closed by #10542