* Add isort to pre-commit

* Run isort on all python files (except __init__.py ones)

find networkx -name \*.py -exec pyupgrade --py36-plus {} \;

The copyright and author stuff is not necessary, out-of-date,
and inconsistent.  It takes up visual space and is a pain to
police everyone doing the same thing on the top of the module.

Git handles authorship in a comprehensive and authoritative way.
The LICENSE.txt file applies to all project code.

* use dict comprehensions in kcomponents.py

* use dict comprehensions in test_kcomponents

* use dict comprehensions in test_kcutsets

* use dict comprehensions in test_maxflow

* use dict comprehensions in test_maxflow_large_graph

* use dict comprehensions in flow/utils.py

* use dict comprehensions in weighted.py

* use dict comprehensions in graphml.py

* use dict comprehensions in nx_pylab.py

* use dict comprehensions in relabel.py

* use dict comprehensions in assortavity/mixing.py

* conform to pep8 guidelines in mixing.py

* Minor tweaks to kcomponents to update to v2.0

Since the residual network is the same for every pair of nodes, delegate
sanity checks on source and sink to algorithm implementations. This makes
build_residual_network easier to use in the connectivity package, and in
general.