7.0: fix most flake8 errors

I did not work on the __unported__ modules
There's still an unused local variable but I don't think this is too bad. 
both are strictly equivalent unless an exception is raised during read(), in which case the context manager ensures fdesc is closed, whereas the other version leaves this to the garbage collector, which may delay the closing to later in case you're running on Jython, or maybe Pypy. 

My goal in this PR is *not* to fix all the doubious code in the addons (the use of mkstemp just above the line you mention is such an example, and it will cause a crash on Windows, I think). Just to make the minimal changes in the code for flake8 to be happy and Travis to produce green builds... and also be able to merge these changes to master and 6.1 to avoid doing this tedious work more than needed.