Added Github Atom to env.rst

Also small PEP8 fixes by flake8.


**Unrelated to pull request:** We should also add/discuss some rules about new lines between headings. 
The style guide is pretty descriptive, but it doesn't touch this subject, which results into many files with varying number of new lines between headings. 

My suggestions are:
* Chapter and page title remain unchanged
* Section and sub section should have 2 new lines for extra readibility