* Tighten up psf/black and flake8
* Fix some tests
* Fix some E741
* Fix some E741
* updating DIRECTORY.md
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* Added doctest and more explanation about Dijkstra execution.
* tests were not passing with python2 due to missing __init__.py file at number_theory folder
* Removed the dot at the beginning of the imported modules names because 'python3 -m doctest -v data_structures/hashing/*.py' and 'python3 -m doctest -v data_structures/stacks/*.py' were failing not finding hash_table.py and stack.py modules.
* Moved global code to main scope and added doctest for project euler problems 1 to 14.
* Added test case for negative input.
* Changed N variable to do not use end of line scape because in case there is a space after it the script will break making it much more error prone.
* Added problems description and doctests to the ones that were missing. Limited line length to 79 and executed python black over all scripts.
* Changed the way files are loaded to support pytest call.
* Added __init__.py to problems to make them modules and allow pytest execution.
* Added project_euler folder to test units execution
* Changed 'os.path.split(os.path.realpath(__file__))' to 'os.path.dirname()'
small off by one error. Boundary condition: if len(number) =13 , we would need to check exactly 1 combination, namely number itself. However for i in range(len(number)-13): will iterate 0 times.
