Test Failure

[abid rahman]

Hi,

1 - I had send a PR on histogram backprojection. at that time travis build 
was successful. 

Then i made some modifications for PEP8, but travis build shows errors 
which i don't understand. 
https://travis-ci.org/scikit-image/scikit-image/jobs/6472929.

But i haven't changed any functionality of code and it works fine in my 
system as good as old.

2 - Also, I didn't get the concept of writing of tests for my code. What 
actually the tests do and how should I write a good test code?

Regards
Abid K
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