Travis CI: Update pypy, Add flake8 tests

[cclauss]

Add flake8 tests to find Python syntax errors and undefined names.

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

[mwouts]

Thanks @cclauss . I like this. But we need to implement #73 first, i.e. add an (un)escape_help_call at places where (un)escape_magic is called (and only for Python and Julia cells, R is fine with quotation mark).

[cclauss]

Why are Python 3.6 and 3.7 failing the Travis tests?

[mwouts]

They were failing because

1. the ?next line came from a notebook (the file was created by test_mirror...). We had to first change the mapping of cell text to scripts (Comment Jupyter help calls in Python and Julia scripts #73) first.
2. local install of jupytext was required for testing, as one of the test calls the nbconvert exporters that the package also provides.

Now everything seems alright, thanks @cclauss for your pull request !