Support for Python 3.7

[cztomczak]

According to docs [1] Python 3.7 on Windows builds with Visual Studio 2015, so it's the same as in Python 3.6. Thus there shouldn't be required any signifcant changes to build tools.

Ref:
[1] https://docs.python.org/3.7/using/windows.html#compiling-python-on-windows

[cztomczak]

Getting error when installing Python 3.7.0b4 on Linux using pyenv:
ERROR: The Python ssl extension was not compiled. Missing the OpenSSL lib?.

All requirements were satisfied from the https://github.com/pyenv/pyenv/wiki/common-build-problems page.

[cztomczak]

According to [1], Python 3.7 requires openssl 1.0.2+ which isn't supported by Ubuntu 14.04.

[1] travis-ci/travis-ci#9069

[cztomczak]

It seems that the only viable solution is to use Ubuntu 16.04 for Python 3.7. So I will have to create virtualbox machines for Python 3.7 32-bit and Python 3.7 64-bit. My current Linux 32-bit virtualbox machine is using 8.7 GB / 12.5 GB disk space (3 GB cefpython build directory alone), so it should be safe to allocate 12 GB for the new machines as well.

Update: Ubuntu 16.04 64-bit with Python 3.7 fully configured takes 6.8 GB. When installing, minimum space requirements are 8.6 GB.

[cztomczak]

Cython (current version 0.25.2) needs to be updated to version 0.28.4 to support Python 3.7. See related issue #250.

[cztomczak]

Python 3.7 is now supported on all platforms.