mulled: use threading.Event in PrintProgress to avoid unnecessary waiting

[mbargull]

After each involucro invocation that takes time t, we wait t % 60 seconds doing nothing.
With threading.Event.wait we can avoid that easily.

[mbargull]

Switched tox configuration to Python 3.5 due to:

• https://github.com/tox-dev/tox/blob/3.14.1/docs/changelog.rst#deprecations-removal-in-next-major-release
• https://github.com/pypa/pip/blob/19.2/NEWS.rst#deprecations-and-removals

[mvdbeek]

Nice, thanks @mbargull. I've cherry-picked this into galaxyproject/galaxy#9163.
Code in https://github.com/galaxyproject/galaxy/tree/dev/lib/galaxy/tool_util is being released as galaxy-tool-util on pypi, which should be more or less a drop-in replacement minus some irrelevant things for third-party projects. galaxy-lib is (pretty much) deprecated, I am not sure we'll do another release.

[mbargull]

Thanks @mvdbeek!
I'll add galaxy-util and galaxy-tool-util to Bioconda in the next few days then.