PEP8: W291 trailing whitespace
imports: update to isort 3.5.0

This enables:
- Specifying lines after import section, 2 in our case
- Allowing import x as y to be sorted together with other imports
imports: two lines after import block
imports: put 'import x as y' in same group
imports: fix some vertical sorts
cpo: test only on Linux

Seems our platform checking wasn't working and it was always skipping.
This should ensure that the test run on Linux and don't run on ther
platforms.
docs: Bump minimum Sphinx version to 1.2.0

Earlier versions either don't work or fail on some of our docstrings.
cpo: skip cpo testing on non-linux 2nd try

The fix in d478e076b8 caused massive amounts of tests to be skipped.  So
revert using the importerskip function and a correct expression.