Restore compatibility with Kwant 1.4

Merged Christoph Groth requested to merge kwant_1.4 into master

This restores compatibility with current Kwant stable (that is to be released as Kwant 1.4.1 rather soon). Compatibility with previous Kwant versions is maintained.

Please also see the commit message.

When using Tkwant with Kwant 1.4 there will be many KwantDeprecationWarnings because Tkwant internally still uses the now deprecated args mechanism. One way to silence these warnings is to run scripts using python3 -Wignore:"The 'args' parameter" or setting the environment variable PYTHONWARNINGS to ignore:The 'args' parameter.

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