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travis-ci

Merge pull request #138 from billsacks/add_python38_tests

Fixes for python3.8, and tweak travis-ci testing

- Critical fix needed for python3.8
- pylint fix needed for all versions of python 3
- Add python3.7 and python3.8 testing through travis-ci
- Remove travis-ci testing on Mac OS (which was using python2, and was
  failing)

User interface changes?: No

Fixes ESMCI/cime#135 ("dictionary keys changed during iteration" when
running checkout_externals)

Testing: 'make lint', 'make test'
  test removed: travis-ci testing on Mac OS
  unit tests: pass
  system tests: pass
  manual testing:

With these diffs in a CESM checkout:

```diff
diff --git a/Externals.cfg b/Externals.cfg
index b943c25..6b0f03e 100644
--- a/Externals.cfg
+++ b/Externals.cfg
@@ -39,7 +39,7 @@ required = True
 tag = release-cesm2.0.03
 protocol = git
 repo_url = https://github.com/ESCOMP/mosart
-local_path = components/mosart
+local_path = ./components/mosart
 required = True

 [pop]
```

ensured that `manage_externals -S` gives the exact same output now as
before.

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