bugfix: add codecov upload back to travis.yml (#10220)

Codecov upload was accidentally removed in #9805

.travis.yml

@@ -207,6 +207,11 @@ script:
 
 after_success:
   - ccache -s
+  - if [[ "$TEST_SUITE" == "unit" || "$TEST_SUITE" == "build" ]]; then
+        codecov --env PYTHON_VERSION
+                --required
+                --flags "${TEST_SUITE}${TRAVIS_OS_NAME}";
+    fi
 
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