Port to Python3

[a-andreyev]

Fixes #28. Initial move with 2to3, manual changes with package and dependent packages rebuilding check.

[Kaffeine]

Mostly a note for myself

In this configuration CMake unable to find python-3 on my system. find_package(PythonInterp ${REQUIRED_PY} REQUIRED) finds python 3 correctly, but I'm not sure if it is a correct replacement. The project compiles on works correctly.

[a-andreyev]

Proposing to add both PythonLibrary and PythonInterp, checked with my configuration, seems ok. (Have problems with PYTHON_SHORT_VERSION variable without PythonLibrary req)

[Kaffeine]

https://cmake.org/cmake/help/latest/module/FindPythonInterp.html says: Deprecated since version 3.12: Use FindPython3, FindPython2 or FindPython instead.

https://cmake.org/cmake/help/latest/module/FindPython.html says: This module looks preferably for version 3 of Python. If not found, version 2 is searched. To manage concurrent versions 3 and 2 of Python, use FindPython3 and FindPython2 modules rather than this one.

It seems that we need to use FindPython3 to ensure the correct python version.

[Kaffeine]

I just verified that the generated code is almost identical to the python2 version. The only diff is in headers:

+<?xml version="1.0" ?><tp:spec ...>
-<?xml version="1.0" encoding="utf-8"?><tp:spec ...>

that is hundred percent fine :+1:

[a-andreyev]

thanks! checked with FindPython3, working for me, updated the commit

[Kaffeine]

Guess what? https://github.com/TelepathyIM/telepathy-glib/commit/ff1eea1e3abdc78665bb8b72c65bb19c8b1084b3#diff-ff4e2dc4962dc25a1512353299992c8dR9 :open_mouth:

While verifying the python3 port I found that we didn't (ever?) build with glib tests support. I think that in the long term we'll drop glib stuff from telepathy-qt (AFAIU it is used for service side because telepathy-qt had no service side API). However, I would like to keep it working as much as possible for the next release. I updated tools from telepathy-glib (hey, we even have a cmake target for that!) and now we need to rebase your PR with the follow little adjustments:

• message(STATUS) is not needed anymore, because now we have FeatureSummary.
• it would be better to keep in sync with telepathy-glib so please prefer the existing version of xincludator.py.
• Please join CMake commits (as they don't make much sense to be applied individually)
• Please fix dbus-python detection. After switching to find_package(Python3) we have no PYTHON_EXECUTABLE variable. I would suggest to introduce our own variable, rather than change the usage of existed one; who knows, maybe one day it will become Python4.
• AFAIU we don't compile our own C/C++ Python extensions, so I think that actually, we don't need PythonLibrary.

I think that the code should look like this one (this is what I tested):

 # Find python version >= 3
find_package(Python3 REQUIRED)

set(PYTHON_EXECUTABLE ${Python3_EXECUTABLE})

 # Check for dbus-python