Trace.taper(): Remove 'slepian', add 'dpss' window

[megies]

What does this PR do?

Removes 'slepian' from Trace.taper() scipy plugins points as it was removed in scipy 1.6 and add 'dpss' as replacement. Don't know how to exactly mimic old slepian with new dpss function so can't prevent user code breaking, though, but at least a replacement is in place for people that used it.

Why was it initiated? Any relevant Issues?

fixes #3331

PR Checklist

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