Copy TLE file to archive at most once

[gerritholl]

Make sure that a TLE file gets copied to the archive at most once, by stopping processing when a regular expression matches. This has the potential to break backward compatibility, but I think it's unlikely that parsing a regular expression based on a partial match is desired, as the additional archival directory names would be incorrect. In the process, I have refactored the module tle_satpos_prepare.py, added the first test case for pytroll-aapp-runner, adapted .travis.yml, and updated the Python version to the modern era.

• [x] Fixes #10
• [x] Tests added
• [x] Tests passed (verified locally as no Travis/Github CI configured)

[ghost]

DeepCode's analysis on #253d3a found:

• :information_source: 2 minor issues. :point_down:

  Top issues

Description	Example fixes
Undefined variable 'breakpoint' Occurrences: test_tle_satpos_prepare.py:68	:wrench: Example fixes
Either all return statements in a function should return an expression, or none of them should. Occurrences: test_tle_satpos_prepare.py:56	:wrench: Example fixes

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[gerritholl]

@pytroll/core Maybe someone with owner permissions on pytroll-aapp-runner can enable Travis CI? There was actually a .travis.yml already before this PR but no unit tests. I'm adding the first unit test here and having CI on Github would be useful.

[gerritholl]

Can someone with triage permissions request reviews from @TAlonglong and @adybbroe? Those seem to have done the most work on this module so far.

[mraspaud]

done

[adybbroe]

Great, thanks @gerritholl for contributing and bringing this repo closer to the modern era! :-)

[adybbroe]

I will merge, and start getting this latest version up and running at SMHI. Then we will see how well it fits our environment.