Update_cmds fixes for py3

[taldcroft]

Fixes for kadi update_cmds to run correctly on Py3. Also includes minor flake8 fixes. Just one non-trivial fix related to the diffing and a bytes/unicode issue. (str() of a bytestring gives b'blah' not blah.)

Testing

On HEAD (kady) in ska3/flight

ska  # LEGACY ska
cd ~/tmp/kadi-test
python /proj/sot/ska/share/update_cmds.py --start=2015:001 --stop=2015:030
# Creates local cmds.h5 and cmds.pkl for the date range using the Ska2-flight kadi.

On local machine (MacOS) in py3-compat branch

ska3  # Activate ska3 environment
cd ~/git/kadi
git checkout py3-compat
cp ~/git/ska_testr/packages/kadi/write_events_cmds.py write_cmds.py

• Edit write_cmds.py to comment out writing the events and fix a Py3-compatibility
  problem (os.path cannot take None in Py3)

rm cmds.*
./update_cmds.py --start=2015:001 --stop=2015:030 --mp-dir ~/ska/data/mpcrit1/mplogs
python write_cmds.py  --start=2014:001 --stop=2016:001
# Creates cmds.txt which is a plain-text version of the commands

mkdir cmds-ska2
cd cmds-ska2
scp 'kady:tmp/kadi-test/cmds.*' ./
python ../write_cmds.py --start=2014:001 --stop=2016:001

diff cmds.txt ../cmds.txt
# Compare cmds from local branch to ska2-flight version
#  ** NO DIFF **

[taldcroft]

Previous logic was that val might be a Py2 str or unicode, and it was necessary to coerce to str. See the dump above of an example of the data structure. In Py3 none of this needed and instead we just try using the numpy item() method to coerce to pure-Python.

[taldcroft]

This is tested (and the testing documented) to my satisfaction, so merging. Not clear yet if this will go as a release or if we should bundle #143 (which is a much bigger change and requires django 2.2).