ENH: Upgrade to Python 2.7.10

This commit upgrades Slicer's Python from version 2.7.3 to version
2.7.10. The newer Python version fixes a number of build warnings seen when
building Slicer with Python 2.7.3 in Visual Studio 2013, like:

C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\xtgmath.h(190): warning C4273: '_hypot' : inconsistent dll linkage [C:\dev\SN\VTKv6-build\Wrapping\PythonCore\vtkWrappingPythonCore.vcxproj]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\math.h(538) : see previous definition of '_hypot'

and:

C:\dev\SN\python-2.7.3\include\pymath.h(22): warning C4273: 'round' : inconsistent dll linkage [C:\dev\SN\python-build\CMakeBuild\libpython\libpython-shared.vcxproj]
    C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\include\math.h(516) : see previous definition of 'round'

In addition to specifying the Python source directory with SRC_DIR, this
commit makes the Python build more robust to default option changes by
explicitly specifying -DDOWNLOAD_SOURCES:BOOL=OFF and
-DINSTALL_WINDOWS_TRADITIONAL:BOOL=OFF.

Also note that 'Lib/distutils/msvc9compiler.py' did not change between Python
2.7.3 and Python 2.7.10, so the patched version remains the same.

This update has been manually tested on:

• Windows (Visual Studio 2008, Release)
• Windows (Visual Studio 2013, Debug)
• Ubuntu 14.04.3 (Debug)
• Mac OS X 10.6.8 (Release)

Release builds show the following tests as failing:

• vtkMRMLSceneImportIDModelHierarchyConflictTest
• py_StandaloneEditorWidgetTest
• py_DTINotReproducibleIssue3977
• qSlicerMultiVolumeExplorerModuleWidgetGenericTest

Debug builds show the following additional test as failing:

• vtkMRMLSceneImportTest_diffusionEditor.mrml

From: Max Smolens <max.smolens@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24526 3bd1e089-480b-0410-8dfb-8563597acbee