* Add isort to pre-commit

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

* Don't use graph6 with directed graphs (#5443)

* Parametrize graph6 tests

* Test graph6 on multigraphs

* Refactor testing utilities

Change `assert_edges_equal`, `assert_graphs_equal`, and `assert_nodes_equal`
to be more pytest-idiomatic.
For example, `assert_edges_equal` becomes the Boolean function `edges_equal`
and then the assert is done the testing file
(i.e., `assert edges_equal(edges1, edges2)`).
This also makes these utility functions useful in nontesting situations
where you want to compare edges, but not raise an exception based on the result.

* Move testing utility functions

* Use new testing utilities

* Deprecate assert_*_equal testing utilities

* Document node, edge, and graph equality helper functions

* text nits.

* Update networkx/tests/test_convert_pandas.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/readwrite/tests/test_sparse6.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/readwrite/tests/test_graph6.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/generators/tests/test_classic.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/algorithms/tree/tests/test_operations.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/algorithms/tree/tests/test_coding.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/algorithms/tests/test_dag.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* Update networkx/algorithms/minors/tests/test_contraction.py

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

* add short equality description to docstring

* Suppress known warnings

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>
Co-authored-by: Dan Schult <dschult@colgate.edu>

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

* Enable tests

* No need to use TestCase