Travis CI: lint Python for syntax errors and undefined names

[cclauss]

In Travis CI, add a Python linting step that runs flake8 to find syntax errors and undefined names.

flake8 testing of https://github.com/IBM/FfDL on Python 3.7.0

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./etc/examples/c10d-onnx-mpi/model-files/train_dist_onnx_mpi.py:93:55: F821 undefined name 'bsz'
    num_batches = ceil(len(train_set.dataset) / float(bsz))
                                                      ^
./etc/examples/c10d-dist-onnx/model-files/train_dist_onnx.py:121:14: E999 SyntaxError: positional argument follows keyword argument
              ', epoch ', epoch, '. avg_loss: ',
             ^
1     E999 SyntaxError: positional argument follows keyword argument
1     F821 undefined name 'bsz'
2

E901,E999,F821,F822,F823 are the "showstopper" flake8 issues that can halt the runtime with a SyntaxError, NameError, etc. Most other flake8 issues are merely "style violations" -- useful for readability but they do not effect runtime safety.

• F821: undefined name name
• F822: undefined name name in __all__
• F823: local variable name referenced before assignment
• E901: SyntaxError or IndentationError
• E999: SyntaxError -- failed to compile a file into an Abstract Syntax Tree

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[Tomcli]

LGTM