Apply pylint to build/soong/bloaty

1. Run black --line-length 80 -S build/soong/bloaty
to fix formatting
2. Annotate # pylint: disable=import-error to skip checks for imports.
The imports are provided by Soong during m <target>

Test: m bloaty_merger_test
Test: pylint --rcfile tools/repohooks/tools/pylintrc
build/soong/bloaty/bloaty_merger.py
build/soong/bloaty/bloaty_merger_test.py
Bug: 195738175

Change-Id: I4579a80203de41d48992424f264dd1cdbafc854c