Update automate.py to work with Python 3

[cztomczak]

The automate.py tool depends on upstream automate-git.py which works only with Python 2.7. To fix it we can use the Python interpreter available from depot_tools. Downloading depot_tools is currently automated within automate-git.py, so the path code for doing the download would need to be extracted from automate-git.py and implemented in automate.py.

Depot_tools on Linux do not ship Python interpreter, only on Windows it is available in depot_tools.

Note that automate.py for now is only intended to build upstream CEF with CEF Python patches applied. You can do this with python 2.7 and the rest in Python 3. You can build the actual cefpython module with Python 3, the compile.py script is compatible with Python 3, same for the make-setup.py to create installer.

[cztomczak]

automate.py --prebuilt-cef runs on Python 3 OK. The --build-cef flag still doesn't work with Python 3.

[cztomczak]

Upstream CEF automate-git.py now supports Python 3. Needs testing.