[python] Increase UT coverage threshold to 0.9 (#8660)

This patch increase Python API unit test coverage
threshold from 85% to 90%. If coverage under 90%
will failed our the test

dolphinscheduler-python/pydolphinscheduler/.coveragerc

@@ -29,4 +29,4 @@ skip_covered = True
 show_missing = True
 precision = 2
 # Report will fail when coverage under 90.00%
-fail_under = 85
+fail_under = 90