Migrate to Python 3

[reszelaz]

This PR migrates Sardana from Python 2 to Python 3.

Sardana version gets bumped to 3.0.0 alpha.

From now on Sardana requires at least:

• Taurus 4.5 Taurus 4.5.5a
• PyTango 9.2.5
• Python 3.5

MacroServer environment created with Python 2 needs to be manually migrated to Python 3.
Pleas use the scripts/upgrade/upgrade_env.py script to do so.

In the migration process we found a serious problem with the Taurus and PyTango, more details in: tango-controls/pytango#292.
Whenever this issue gets clarified we will need to revert 2f0a36a.

Also whenever taurus-org/taurus#945 gets clarified we will need to adapt usage of JSONCodec (related to 25dce83). This is already done.

I take a profit of this message to ask @sardana-org/integrators and anyone else interested in helping to test this PR. Whenever you find an issue with this branch please report it starting the issue summary with "Py3: ". If you have any question about the migration either comment on #1089 or open a dedicated issue for that.

Before integration we need to perform the release manual tests. I will try to extend the manual test scenarios to cover more features which are not tested automatically.

We plan to test the following platforms:

• Debian 9
• Centos 7
• Windows 10

The reszelaz/sardana-test:py3 docker image or the following conda environment can serve as inspiration for preparing the testing environment:

conda config --add channels conda-forge
conda config --add channels tango-controls
conda create -n sdn-py3
# optional if someone install in user site: export PYTHONNOUSERSITE=1
conda activate sdn-py3
conda install python=3.5 pyqt=5 itango pytango lxml future guidata ipython pillow pint ply pyqtgraph pythonqwt numpy scipy click h5py matplotlib
pip install --no-deps -e . # for taurus and sardana

Many thanks in advance for help in the migration process!

Fixes #1089.

[reszelaz]

• Manual tests on Debian 9 (Py3: manual tests on Debian 9 #1175) revealed some problems which are already fixed. These tests included the standard release manual tests plus some extra test scenarios.
• The Windows platform can not be tested since we lack of working conda environment - we need to wait for PyTango 9.3.1 conda package due to Spock blocks when using tango 9.3 #1152.
• The original plan was also to test it on CentOS 7 before integration of this PR into develop.

What do you think if we consider that the Debian 9 tests are enough? And we test both CentOS 7 and Windows ASAP but already after integrating this PR? Could you please express your opinions, especially @sardana-org/integrators ?

This is a summary of issues of high importance that we need to have in mind:

1. Your custom plugins (controllers/macros/recorders/widgets using Taurus extension) needs to be ported to Python 3.
2. The MacroServer environment created with Python 2 needs to be ported to Python 3: see sardana/scritps/upgrade/upgrade_env.py.
   This requires a choice of the shelve backend (either gnu or dumb) at the moment of migration. The same decission is also important for new Sardana systems. In this last case it can be tuned with this sardanacustomsetting.MS_ENV_SHELVE_BACKEND.
3. Online scan trend does not work yet in this branch - Qwt5 is not available for Python 3. We plan to use taurus_pyqtgraph and will start to implent this ASAP. Already implemente in Make online scan work with pyqtgraph reszelaz/sardana#25.

There are also some technical debts that we took already and we need to pay ASAP:

1. We were forced to not use taurus.Device with the Sardana extensions in some places of the MacroServer (see commit 2f0a36a) due to DS crashes when unsubscribing from events in __del__ tango-controls/pytango#292.
2. Device_4Impl.delete_device is not called due to Import taurus in PyTango DS causes that delete_device is not called at shutdown taurus-org/taurus#982.

[teresanunez]

Hi @reszelaz, many thanks for all this work, I agree with you that it can be integrated.

[amilan]

As agreed in today's sardana follow-up meeting, this PR is ready to be merge.