Fix test suite

[cpascual]

The test suite is still too fuzzy, and the py35-qt5 target had to be quarantined.

This PR does a large refactoring of the tests (mostly the taurus/qt ones) to improve the situation. With it, the testsuite seems finally stable for all the targets. As a summary, here is what changed:

• refactor all the qt tests to use pytest-qt (this seemed to fix many of the "random" crashes during test execution). And deprecate old unittest-based qt test utilities and TimeOut DS.
• force to use PyTango >=9.3.1 for all targets (I had to provide my own conda package of PyTango v9.3.1 which was not officially available, see ab6888b )
• Add some pytest-based utilities for new tests
• Fix a few bugs in the code that affect the tests (8efbe87 , 14d94b8, e48a2d9, dcb6831, )
• Remove taurus usages of deprecated APIs of dependencies (1d44c7a, 46ddb2c, b5173ad8, 964b18fa, 6cb42ff, 4aaf110a, 5bc24ad) and configure the tests to treat future occurrences as errors (66e4d32)
• change the tango docker images to use a mirror from those of ska-docker to avoid availability issues of ska's registry

Also, this PR removes the allowed-to-fail tests (python2 ones and flake8) from travis matrix (to avoid wasting time). They can be run locally and/or re-enabled at any moment

[cpascual]

any @taurus-org/integrators daring to have a look at this PR? ;) It would be nice to integrate it soon to unblock the road for the other changes needed for the Jul20 release

[tiagocoutinho]

Ok, for what is worth I did an overview and it LGTM.

Also ran the tests locally (all except epics): Ran 565 tests in 6.247s

The CI likes it also go for it! Do you press the button or should I?

[cpascual]

Thanks @tiagocoutinho !

Do you press the button or should I?

do the honors yourself!