Tests for docs being up-to-date should not be part of functional tests

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Currently we test with py.test whether headers and docs reflect the current year/month.

When a year/month marker in docs or source headers outdates, then this is a problem for packagers and release maintainers. It is not a problem for people running the code.

We might therefore do this kind of checks with tox, but in a different manner.

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Further findings: the mentioned tests fail automatically after a year (or shorter). They should fail only, if there were any commits after the year mentioned in docs and version info.