Make TaskSet .user and .parent read only properties, avoids / fixes #1379

[cyberw]

No description provided.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@387f44d). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1380   +/-   ##
=========================================
  Coverage          ?   81.32%           
=========================================
  Files             ?       26           
  Lines             ?     2329           
  Branches          ?      360           
=========================================
  Hits              ?     1894           
  Misses            ?      343           
  Partials          ?       92           

Impacted Files	Coverage Δ
locust/user/task.py	96.72% <100.00%> (ø)
locust/util/cache.py	91.30% <0.00%> (ø)
locust/event.py	90.00% <0.00%> (ø)
locust/argument_parser.py	76.14% <0.00%> (ø)
locust/user/sequential_taskset.py	85.18% <0.00%> (ø)
locust/rpc/__init__.py	100.00% <0.00%> (ø)
locust/contrib/fasthttp.py	87.92% <0.00%> (ø)
locust/rpc/zmqrpc.py	86.66% <0.00%> (ø)
locust/util/exception_handler.py	80.00% <0.00%> (ø)
... and 17 more

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