Cherrypy

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* :pr:`1580`: ``CPWSGIServer.version`` now reported as
``CherryPy/x.y.z Cheroot/x.y.z``. Bump to cheroot 5.2.0.
* The codebase is now :pep:`8` complaint, flake8 linter is `enabled in TravisCI by
default <https://github.com/cherrypy/cherrypy/commit/b6e752b>`_.
* Max line restriction is now set to 120 for flake8 linter.
* :pep:`257` linter runs as separate allowed failure job in Travis CI.
* A few bugs related to undeclared variables have been fixed.
* ``pre-commit`` testing goes faster due to enabled caching.

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* :issue:`1342`: Fix AssertionError on shutdown.

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* Bump to cheroot 5.1.0.

* :issue:`794`: Prefer setting max-age for session cookie
expiration, moving MSIE hack into a function
documenting its purpose.

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* :issue:`1332`: CherryPy now uses `portend
<https://pypi.org/project/portend>`_ for checking and
waiting on ports for startup and teardown checks. The
following names are no longer present:

- cherrypy._cpserver.client_host
- cherrypy._cpserver.check_port
- cherrypy._cpserver.wait_for_free_port
- cherrypy._cpserver.wait_for_occupied_port
- cherrypy.process.servers.check_port
- cherrypy.process.servers.wait_for_free_port
- cherrypy.process.servers.wait_for_occupied_port

Use this functionality from the portend package directly.

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* :issue:`1481`: Move functionality from cherrypy.wsgiserver to
the `cheroot 5.0 <https://pypi.org/project/Cheroot/5.0.1/>`_
project.

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* :issue:`1537`: Restore dependency on pywin32 for Python 3.6.