Fix flake8 issues since release 3.6.0
Flake8 had a new release which raise new issues, namely:
W504: line break after binary operator
F841: local variable 'ex' is assigned to but never used
W605: invalid escape sequence
F821: undefined name 'buffer' (noqa seems the only way to avoid this false positive)
Also pin flake8>=3.6 in our tests and make explicit that we use python3 to run
flake8 tests.
.. _bwcompat_module:
:mod:`cubicweb.pyramid.bwcompat`
--------------------------------
.. automodule:: cubicweb.pyramid.bwcompat
.. autofunction:: includeme
.. autoclass:: PyramidSessionHandler
:members:
.. autoclass:: CubicWebPyramidHandler
:members:
.. automethod:: __call__
.. autoclass:: TweenHandler
:members: