slurm.py error and bug fix

[DS-Wen]

Found an error when using pstat to check my job progress.

The error message was:
Traceback (most recent call last):
File "/home/wen94/.conda/envs/cent7/5.3.1-py37/casm/bin/pstat", line 11, in
load_entry_point('prisms-jobs==4.0.2', 'console_scripts', 'pstat')()
File "/home/wen94/.local/lib/python3.6/site-packages/prisms_jobs/scripts/pstat.py", line 283, in main
db.update()
File "/home/wen94/.local/lib/python3.6/site-packages/prisms_jobs/jobdb.py", line 301, in update
active_status = config.software().job_status()
File "/home/wen94/.local/lib/python3.6/site-packages/prisms_jobs/interface/slurm.py", line 233, in job_status
jobstatus["qstatstr"] += line
TypeError: unsupported operand type(s) for +=: 'NoneType' and 'str'

After commenting out the problematic line "jobstatus["qstatstr"] += line" in slurm.py, pstat works fine.