Remove ds9 warnings when run under Python 3.6

[DougBurke]

Release Note

Update the DS9 code so that external processes are cleaned up properly, so to remove the potential ResourceWarning warnings when running DS9 on Python 3.6.

Notes

Update the DS9 code so that external processes are cleaned up properly. This removes the ResourceWarning warnings raised when running the sherpa/image/tests/test_image.py test. The Travis python 3.6 build is no longer allowed to fail.

[DougBurke]

This replaces PR #364. It removes the decorator and --show-warnings option.

[DougBurke]

Should we update the python-support button in README.md to now say "2.7,3.5,3.6" (either as part of this PR or once this has landed)?

[olaurino]

would the commit I added do?

[DougBurke]

LGTM - you can see the badge at https://github.com/DougBurke/sherpa/blob/remove-ds9-warnings-with-py36/README.md

There's some other clean up needed in README.md, but I think it's more extensive than is warranted for this PR (e.g. there's mention of the latest release being 4.8.2), and should be done in a "Here's the documentation for the 4.9.1 release" PR).

[olaurino]

Unfortunately, this code does not work in Python 2, where Popen does not implement the context manager protocol. I am exploring some options, but maybe we should just revert this? The real problem is that we didn't pick up the issue from the CI jobs, so I am making sure we add a ds9 smoke test, and that we require ds9 in the CI jobs that install it as a dependency.

[DougBurke]

I guess the

with Popen(...) as blah:
    moreblah()

could be changed to

blah = Popen(...)
try:
    moreblah()
finally:
    blah.<not sure exactly what methods to call here>()

but, as indicated, I'm not sure what needs to be explicitly closed.

Another option would be to add the ResourceWarning messages to conftest.py so that we can pass the tests with Python 3.6 (as these particular warnings are not "serious").

[DougBurke]

Or there are some 2.7 solutions in https://stackoverflow.com/questions/30421003/exception-handling-when-using-pythons-subprocess-popen

I think we could define _Popen in Python pre 3.2 to have the necessary enter/exit methods to act as a context manager.

[DougBurke]

With python 2.7 I get this:

% python -c 'import sherpa.astro.ui'
WARNING: imaging routines will not be available, 
failed to import sherpa.image.ds9_backend due to 
'AttributeError: __exit__'
WARNING: failed to import sherpa.astro.xspec; XSPEC models will not be available

as a quick check to see if it's still happening.

[DougBurke]

See PR #373