Travis CI: Add pytest --doctest-modules machine_learning (#1016)

* Travis CI: Add pytest --doctest-modules neural_network

Fixes #987
```
neural_network/perceptron.py:123: in <module>
    sample.insert(i, float(input('value: ')))
../lib/python3.7/site-packages/_pytest/capture.py:693: in read
    raise IOError("reading from stdin while output is captured")
E   OSError: reading from stdin while output is captured
-------------------------------------------------------------------------------- Captured stdout --------------------------------------------------------------------------------
('\nEpoch:\n', 399)
------------------------

value:
```

* Adding fix from #1056 -- thanks @QuantumNovice

* if __name__ == '__main__':

* pytest --ignore=virtualenv  # do not test our dependencies