Uses target python3 on conditional buids, fixes #1485

[AndreMiras]

python3crystax support was removed from the CI in #1471.
Conditional builds now relies on python3 with fallback to python2.

Adds python3 support to jedi recipe to demonstrate.

[AndreMiras]

Build is green on my fork https://travis-ci.org/AndreMiras/python-for-android/builds/466184240
Conditional build raw log: https://api.travis-ci.org/v3/job/466184245/log.txt
Relevant log extract:

...
[INFO]:    Found multiple valid dependency orders:
[INFO]:        ['hostpython2', 'python2', u'jedi']
[INFO]:        ['hostpython3crystax', 'python3crystax', u'jedi']
[INFO]:        ['hostpython3', 'python3', u'jedi']
[INFO]:    Using the first of these: ['hostpython2', 'python2', u'jedi']
[INFO]:    Trying to find a bootstrap that matches the given recipes.
...
[INFO]:    Found 4 acceptable bootstraps: ['webview', 'service_only', 'sdl2', 'pygame']
[INFO]:    Using the first of these: webview
[INFO]:    Found a single valid recipe set: ['hostpython2', 'python2', u'jedi']
[INFO]:    -> directory context testapps/
('recipes modified:', set([u'jedi']))
('recipes to build:', set([u'jedi']))
incompatible with python3
falling back to python2
('recipes to build (no broken):', set([u'jedi']))
('requirements:', u'jedi,python2')
...
           working: BUILD SUCCESSFUL in 41s                                                        �[0m
...
�[1m[INFO]�[0m:    # APK renamed to bdisttest_python2-1.1-debug.apk
...
Done. Your build exited with 0.

For some reason the conditional build script marked it as incompatible with python3 and fell back to python2, but that's something to investigate in a different task to be investigated once that one is merged.
For now on at least the build doesn't crash with a KeyError: 'CRYSTAX_NDK_HOME'.

[ghost]

I don't know much about the CI setup of p4a, but just from looking over the code I can't see anything that is obviously nonsense or questionable

[inclement]

It isn't really documented anywhere, but you can/should omit this from the dependencies in which case all python recipes will be added automatically.

[AndreMiras]

Good to know thanks!