Python > 3.7 error with libcloud and postgresql fd plugin

[franku]

With Python > 3.7 the libcloud and postgresql plugins do not work anymore.

The suggested change in this PR is to write an error message to the joblog and cancel the job.

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[joergsteffens]

I've tested the pyplug-fd-postgres on different platforms and it works/fails as described.

[joergsteffens]

The pyplug-fd-postgres part looks good.