Use pytest instead of nose

[sebastic]

As reported in Debian Bug #1018529, nose is an obsolete testing framework for Python, dead and unmaintained since 2015 [1][2], nose2 or pytest should be used instead.

Only test/run_tests.py still uses nose, and was used for Travis CI in the past, but it doesn't seem to be used any more. I think the script can just be removed.