Switch test runner from nosetest to pytest (#6291) #5625

Job	Run time
Skip Duplicate Jobs Pre Job	6s
Self-check on Python 3.8 - Python 3.8	8m 34s
Lint Checks (black, flake8, etc.) - Python 3.8	2m 46s
Pack Tests - Python 3.8	2m 38s
Compile (pip deps, pylint, etc.) - Python 3.8	3m 49s
Unit Tests (chunk 1) - Python 3.8	10m 17s
Integration Tests (chunk 1) - Python 3.8	7m 31s
Lint Checks (black, flake8, etc.) - Python 3.9	2m 47s
Unit Tests (chunk 2) - Python 3.8	11m 1s
Integration Tests (chunk 2) - Python 3.8	6m 56s
Compile (pip deps, pylint, etc.) - Python 3.9	4m 37s
Unit Tests (chunk 1) - Python 3.9	10m 20s
Pack Tests - Python 3.9	2m 50s
Unit Tests (chunk 2) - Python 3.9	10m 36s
Integration Tests (chunk 1) - Python 3.9	7m 17s
Integration Tests (chunk 2) - Python 3.9	6m 12s
Slack notification for failed master builds	4s
	1h 38m 21s

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