Auto format using Black (20.8b1) Added black --check to build.

[cyberw]

Fixes #1489 (kinda)

My second attempt at this (replacing #1494), now that magic trailing commas has been fixed in Black.

• Black is very popular, so many will be familiar with it.
• It is good at minimizing diffs
• It is very opinionated (the only config I've added is the line length), so there should be minimal discussion about how to configure it
• IMO it strikes a good balance between keeping the code nicely formatted (and PEP8 compliant) without making it unnecessarily long.

Here's a comparison of various formatters https://medium.com/3yourmind/auto-formatters-for-python-8925065f9505

As the next step, we could (should) increase the number of rules covered by flake8 (probably switching to ignoring the few rules that we dont like instead of enabling a few favourite ones. something like --ignore E501,W503,E701,E731,E741,E712,E402,F401).

[codecov]

Codecov Report

Merging #1546 into master will decrease coverage by 0.15%.
The diff coverage is 73.28%.

@@            Coverage Diff             @@
##           master    #1546      +/-   ##
==========================================
- Coverage   82.50%   82.34%   -0.16%     
==========================================
  Files          28       28              
  Lines        2566     2566              
  Branches      391      391              
==========================================
- Hits         2117     2113       -4     
- Misses        353      364      +11     
+ Partials       96       89       -7     

Impacted Files	Coverage Δ
locust/__init__.py	100.00% <ø> (ø)
locust/exception.py	100.00% <ø> (ø)
locust/rpc/protocol.py	100.00% <ø> (ø)
locust/user/sequential_taskset.py	85.18% <0.00%> (ø)
locust/main.py	21.05% <15.62%> (ø)
locust/argument_parser.py	77.19% <33.33%> (ø)
locust/user/inspectuser.py	72.72% <37.50%> (ø)
locust/event.py	87.50% <50.00%> (ø)
locust/util/deprecation.py	90.47% <66.66%> (ø)
locust/util/exception_handler.py	80.00% <66.66%> (ø)
... and 19 more

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[cyberw]

doh, wrong branch...