Add python3 support

[o01eg]

This PR is the rebased @adrianbroher python3 branch with python 3.5.7 update and some fixes.

Fixes #47 . Supersedes #50

There both python3.{dll,lib,pdb} and python35.{dll,lib,pdb}. Those libraries are different. Is it intended?

[adrianbroher]

There both python3.{dll,lib,pdb} and python35.{dll,lib,pdb}. Those libraries are different. Is it intended?

Probably not, but the used python-cmake is a good mile behind the latest development.

[o01eg]

Probably not, but the used python-cmake is a good mile behind the latest development.

I've updated python-cmake for latest commit, it successfully builds python but python3.{lib,dll} and python35.{lib,dll} still exists both.

[o01eg]

Also for both python2 and python3 support boost should be updated to 1.67:

The library name now includes the version suffix of the Python version used to compile it. For example, a variant compiled with Python 2.7 will produce library names boost_python27 and boost_numpy27, etc.. Combined with a related fix in Boost.Build, this means that it is now possible to build variants for multiple Python versions in a single build process.

But I get strange error:

  ...skipped <pbin.v2\libs\log\build\msvc-14.0\rls\adrs-mdl-64\dbg-str-dtbs\dbg-symbl-on\pch-off\thrdp-wn32\thrd-mlt>attribute_name.obj for lack of <pbin.v2\standalone\msvc\msvc-14.0>msvc-setup.nup...

[o01eg]

If I simply update boost to 1.67 CI only builds boost_python27.{dll,lib}. If I force to build both python with python=2.7 python=3.5 I got errors.

[o01eg]

Rebased on top #49 to test with boost 1.68

[o01eg]

Both python support was fixed in 1.70 but there GIL issues to update to this boost version.

[o01eg]

As freeorion/freeorion#2468 found to be fixed this PR is ready to be merged as well.

[Vezzra]

Anything preventing merge?

[o01eg]

I didn't find issues with this SDK.

[geoffthemedio]

If you think it's good and no-one objects, then I suggest merging...