Switch to pytest in CI and add unstable dependency environment

[djhoese]

This PR follows our general pytroll trend of moving to pytest for running tests. It also addresses the issue in #1059 about having an environment that installs development versions of xarray and dask and other libraries.

• Closes test against xarray master #1059
• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[codecov]

Codecov Report

Merging #1095 into master will increase coverage by 0.36%.
The diff coverage is 99.13%.

@@            Coverage Diff             @@
##           master    #1095      +/-   ##
==========================================
+ Coverage   88.94%   89.31%   +0.36%     
==========================================
  Files         194      195       +1     
  Lines       29823    28749    -1074     
==========================================
- Hits        26526    25677     -849     
+ Misses       3297     3072     -225

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_geocat.py	100% <ø> (ø)	⬆️
satpy/tests/reader_tests/test_slstr_l2.py	100% <ø> (+2.22%)	⬆️
satpy/tests/compositor_tests/__init__.py	100% <ø> (+0.38%)	⬆️
satpy/tests/reader_tests/test_aapp_l1b.py	100% <ø> (ø)	⬆️
satpy/tests/reader_tests/test_iasi_l2.py	98.61% <ø> (+0.59%)	⬆️
satpy/tests/writer_tests/__init__.py	100% <ø> (+6.25%)	⬆️
satpy/tests/writer_tests/test_ninjotiff.py	100% <ø> (ø)	⬆️
satpy/resample.py	90.76% <ø> (+0.13%)	⬆️
satpy/tests/reader_tests/__init__.py	100% <ø> (+1.49%)	⬆️
satpy/tests/reader_tests/test_eps_l1b.py	100% <ø> (ø)	⬆️
... and 98 more

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[coveralls]

Coverage increased (+0.4%) to 89.3% when pulling 6ec63d5 on djhoese:testing-pytest into 33506e2 on pytroll:master.

[djhoese]

CodeFactor is running python 2 checks when it doesn't need to. I'm not sure I feel like making a pylint config just to correct it (I assume it will stop these checks eventually). It is made at me for reusing the builtin reduce name...but in python reduce isn't builtin, it has to be imported from functools...which is what I'm doing.

[djhoese]

The coverage difference worries me. It looks like some of the tests aren't being run any more. First guess is my changes to isinstance where it was using six.text_types.

[mraspaud]

Thanks a lot for the comprehensive clean up! I just found a copy-paste error that needs to be fixed (not you doing though :) )

[sfinkens]

Tremendous! Thanks a lot!

[djhoese]

FYI Martin approved on slack. So merging now.