Fix HDF5 utility file handler not decoding byte arrays consistently

[djhoese]

A couple users noticed that some readers are now causing errors when multiple granule's metadata dictionaries are combined. You'll see errors like:

  File "/home/cpeters/anaconda2/envs/satpy/lib/python3.7/site-packages/numpy/core/numeric.py", line 2287, in isclose
    xfin = isfinite(x)
TypeError: ufunc 'isfinite' not supported for the input types, and the inputs could not be safely coerced to any supported types according to the casting rule ''safe''

I've tracked this down to at least one specific issue: some readers are putting byte arrays (ex. b''"') in numpy arrays in their DataArray.attrs metadata. When attempting to combine metadata satpy sees these as arrays and attempts to combine them. This fails with the above error.

Both readers that have encountered this are using the base HDF5 utility file handler. I discovered that although the file handler is converting attributes to regular strings when possible, it is only doing this for attributes accessed through the string form my_handler['/my_data/attr/some_attr'] but NOT for the DataArray that is produced so doing my_handler['my_data'].attrs['some_attr] will have the unconverted data.

Additionally this fixes the unstable travis environment failures in the HY2 reader.

• Closes #xxxx
• Tests added
• Passes flake8 satpy

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[codecov]

Codecov Report

Merging #1422 into master will increase coverage by 0.00%.
The diff coverage is 92.85%.

@@           Coverage Diff           @@
##           master    #1422   +/-   ##
=======================================
  Coverage   90.58%   90.58%           
=======================================
  Files         236      236           
  Lines       33797    33820   +23     
=======================================
+ Hits        30615    30637   +22     
- Misses       3182     3183    +1     

Impacted Files	Coverage Δ
satpy/readers/hdf5_utils.py	91.42% <77.77%> (-1.00%)	⬇️
satpy/readers/mersi2_l1b.py	97.70% <85.71%> (+1.18%)	⬆️
satpy/readers/hy2_scat_l2b_h5.py	98.00% <100.00%> (ø)
satpy/tests/reader_tests/test_hdf5_utils.py	94.59% <100.00%> (+1.04%)	⬆️
satpy/tests/reader_tests/test_mersi2_l1b.py	99.63% <100.00%> (ø)
satpy/readers/nwcsaf_msg2013_hdf5.py	94.87% <0.00%> (-1.29%)	⬇️
satpy/tests/reader_tests/test_nucaps.py	100.00% <0.00%> (ø)
satpy/readers/nucaps.py	89.18% <0.00%> (+0.04%)	⬆️

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[coveralls]

Coverage increased (+0.003%) to 90.588% when pulling 3765b95 on djhoese:bugfix-hy2-reader into 9ccddbf on pytroll:master.

[coveralls]

Coverage decreased (-0.03%) to 90.551% when pulling fa0f17b on djhoese:bugfix-hy2-reader into 9ccddbf on pytroll:master.

[TAlonglong]

LGTM

[mraspaud]

Thanks for taking care of this, LGTM