Remove event.dataset from integration with elastic-apm

[basepi]

Also removes python 3.5 support as we now test against elastic-apm @ master which doesn't support python 3.5.

Ref elastic/apm-agent-python#1365

[apmmachine]

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• Start Time: 2021-10-14T17:26:41.574+0000

• Duration: 8 min 31 sec

• Commit: 4622cdc

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Test	Results
Failed	0
Passed	162
Skipped	0
Total	162

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[basepi]

/test

[sethmlarson]

LGTM.

Validating dropping Python 3.5 by looking at our PyPI stats, we've received 0-1 downloads from Python 3.5 for the past while. Goodbye Python 3.5!