Fix iterate_messages() generator raising StopIteration

[pnuu]

This PR fixes iterate_messages() what was raising StopIteration when it should just return.

Also adds an unit test for this function, and fixes a flake8 complaint about invalid usage of assert:

trollmoves/tests/test_client.py:86:9: B011 Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().

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[codecov]

Codecov Report

Merging #60 into master will increase coverage by 0.16%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   65.09%   65.26%   +0.16%     
==========================================
  Files          14       14              
  Lines        2630     2637       +7     
==========================================
+ Hits         1712     1721       +9     
+ Misses        918      916       -2     

Impacted Files	Coverage Δ
trollmoves/tests/test_client.py	99.75% <88.88%> (+<0.01%)	⬆️
trollmoves/client.py	67.27% <100.00%> (+0.36%)	⬆️

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[mraspaud]

For the record, this is new behavious since python 3.7:

Changed in version 3.7: Enable PEP 479 for all code by default: a StopIteration error raised in a generator is transformed into a RuntimeError.

[mraspaud]

LGTM