* test case for zebra problem

* Revert "Merge remote-tracking branch 'upstream/master'"

This reverts commit 5ceab1a27ff73b410200c2f84d22e0a1d79b6fbe, reversing
changes made to 34997e48547a57939d568f170c60e442c2d54db5.

* discarded HEAD changes for merge

* added ensemble_learner jpeg

* updated travis and search

* Added logarithmic learner in nlp_apps (#890)

* Remove zebra tests

* revised Bayes learner explanation

* added missing SimpleReflexAgent from upstream

* Added section on min-conflicts

* Refactor one-liner for loop

* Added tests for min_conflicts and NQueensCSP

* added submodule

* fixed ac3 in csp.py

* added a test to verify the modified ac3 algorithm in csp.py

* Update .gitmodules

* add failure_test method to utils

* comment fix

* Update test_learning.py

* Update test_csp.py

* Added tests for search.py

* Updated readme

* Removed extra newline

* Fixed seed

* Update README.md

* Added docstring for minimal-consistent-det

* Update csp.py

* Add test

* Fix flake8 warnings

* Remove unnecessary #noqa

* Fix doctest

* Implement Topological Sort

- Implemented topological_sort
- Added auxiliary function build_topological
- Updated call to topological_sort

* Use defaultdict to build visited

* Added test

* Update csp.py

Skip first item of iteration

Add test for the following functions from csp.py

* revise
* AC3
* first_unassigned_variable
* num_legal_values
* mrv

Add tests to the following methods from CSP class

* result
* goal_test
* support_prunning
* suppose
* prune
* choices
* infer_assignement
* restore
* conflicted_vars

* Add flake8 config file

* Fix flake8 for test files

* Add tests to CSP class

Add test for the following methods on CSP class:

* assign
* unassigns
* nconflits
* actions

* Refactor some asserts in test_csp.py

Some asserts were being done in the following way:
assert (X is not None) == True

Now, they are handled in the following way:
assert X