Fix linting errors in server code

[ptitzler]

Fixes

$ make lint-server
python3 -m flake8 elyra .github
elyra/tests/pipeline/test_handlers.py:33:1: I100 Import statements are in the wrong order. 'from elyra.pipeline.processor import PipelineProcessorManager' should be before 'from elyra.tests.util.handlers_utils import expected_http_error'
elyra/tests/pipeline/test_handlers.py:33:1: I202 Additional newline in a group of imports. 'from elyra.pipeline.processor import PipelineProcessorManager' is identified as Application and 'from elyra.tests.util.handlers_utils import expected_http_error' is identified as Application.
elyra/tests/pipeline/test_handlers.py:36:1: I100 Import statements are in the wrong order. 'from elyra.pipeline.parser import PipelineParser' should be before 'from elyra.pipeline.validation import ValidationResponse'
make: *** [lint-server] Error 1

which was introduced in https://github.com/elyra-ai/elyra/pull/2876/files

What changes were proposed in this pull request?

• Fix import order and grouping

How was this pull request tested?

$ make lint-server

CI task output:

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[elyra-bot]

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[kevin-bates]

LGTM