Drop Python 2 support

[cdeil]

This PR drops Python 2 support (see PIG 3 and #1963).

Here I only do the simple / obvious / partly automated changes. This will result in a large diff.

More specific changes that might require discussion (like e.g. using concurrent.futures.ProcessPoolExecutor instead of multiprocessing.Pool) will follow tomorrow in separate small PRs.

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Following the advice here, I've added python_requires>=3.5 in setup.py, which declares to pip to never install Gammapy>=0.11 on Python 2.

$ pip2 install .
Processing /Users/deil/work/code/gammapy
gammapy requires Python '>=3.5' but the running Python is 2.7.15

I think that's enough, raising errors in setup.py or gammapy/__init__.py as mentioned here is only needed for users that have a pip that is 3+ years old, and try to update to new Gammapy, i.e. a tiny (possibly non-existent) fraction of our users.

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Other changes in this PR:

• Remove Python 2 CI builds
• Update install docs
• Remove gammapy/extern/six.py and adjust all users (manually)
• Remove gammapy/extern/pathlib.py and adjust imports where it's used (manually)
• Remove object as base class (possible using 2to3 or need to do manually?)
• Remove __future__ imports (using 2to3)

[cdeil]

This PR is ready. @adonath or anyone interested - I don't think there's anything that needs review or is controversial here, but if you want, have a look (review one commit at a time should be easy). I'll wait a few hours before merging.

I would suggest to keep #1963 for about another week and discuss / gather suggestions for non-trivial updates (like using ProcessPoolExecutor or keyword-only arguments) there. If anyone finds something that can be modernised or cleaned up in Gammapy now that we're Python 3.5+, or if there is a feature we could start using, please leave a comment there.

[adonath]

Thanks @cdeil! Yes, please go ahead and just merge...we've been waiting for this day already:-)

[adonath]

Looks good to me!