Make stdout/stderr wrappers identify themselves as not being a tty

[michaelboulton]

Calling isatty() on sys.stderr/sys.stdout inside locust will currently raise an exception (AttributeError: 'StdOutWrapper' object has no attribute 'isatty'), this PR just makes it so that it definitely identifies itself as not being connected to a tty.

[codecov]

Codecov Report

Merging #929 into master will decrease coverage by 0.04%.
The diff coverage is 50%.

@@            Coverage Diff            @@
##           master    #929      +/-   ##
=========================================
- Coverage   66.55%   66.5%   -0.05%     
=========================================
  Files          14      14              
  Lines        1438    1442       +4     
  Branches      226     226              
=========================================
+ Hits          957     959       +2     
- Misses        430     432       +2     
  Partials       51      51

Impacted Files	Coverage Δ
locust/log.py	63.88% <50%> (-1.74%)	⬇️

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