Use python3.6 for tox and travis

[tserong]

Commit f8a6269 switched to using generic python3, allegedly because Tumbleweed had python 3.7, not 3.6. I assume that was true at the time, because I wrote it, but given that python 3.6 is currently available on Tumbleweed, perhaps I was mistaken. Anyway, it makes the most sense to force testing with python3.6, because that's what we deploy against with SES6 on SLE 15 SP1.

This commit also:

• Forces the use of isort < 5 when running lint, because version 5 produces a pile of "module 'isort' has no attribute 'SortImports'" errors, and no actual useful lint output.
• Gets rid of the file permission checks when running in Travis CI, because Travis sets umask 002, which messes up the file permissions during git clone, and causes all the tests to fail.

Signed-off-by: Tim Serong tserong@suse.com

[tserong]

Note that it doesn't matter here that the Travis CI builds failed in lint - they're failing less now after this commit than they were before, so things are improved.