Clickable

* cleaned build system
* tests reworked
* updated dependencies
* updated versioning scheme (major/minor)
* python requirement: mandatory 3.6+ (previously 2.7+)
* ruamel.yaml replaced by PyYAML

* remove clickable.bootstrap module
* clickable.click use 'main' as default function when searching
an entry-point in clickables.py
* tasks.py renamed to clickables.yml
* added an helper to load base configuration from clickables.yml
(clickables.utils.load_config)
* python3 support

(delayed release, used from dev branch for 6 months)

* added clickable.bootstrap
* added helpers for sphinx commands
* added workaround for selinux and virtualenv

* fix rsync ``options`` arg behavior

* added rsync handler

* correctly handle clear_env in sphinx:sphinx_script
* update cryptography, tox, sphinx, wheel