Please fix the following 'cros lint' errors:

toolchain-utils/crosperf/download_images_buildid_test.py
************* Module download_images_buildid_test
R:  1, 0: Shebang should be #!/usr/bin/python2 or #!/usr/bin/python3 (bad-shebang)
C:  1, 0: Modules should have docstrings (even a one liner) (module-missing-docstring)
W:  5, 0: Uses of a deprecated module 'optparse' (deprecated-module)
C: 18, 0: Classes should have docstrings (even a one liner) (class-missing-docstring)
C: 83, 2: Second line should be blank (docstring-second-line-blank)
R:  1, 0: Missing "from __future__ import print_function" line (missing-print-function)

toolchain-utils/crosperf/image_checksummer.py
************* Module image_checksummer
C:  1, 0: Modules should have docstrings (even a one liner) (module-missing-docstring)
C: 11, 0: Classes should have docstrings (even a one liner) (class-missing-docstring)
C: 13, 2: Classes should have docstrings (even a one liner) (class-missing-docstring)
R:  1, 0: Missing "from __future__ import print_function" line (missing-print-function)


toolchain-utils/crosperf/mock_instance.py
************* Module mock_instance
W:  8, 0: Unused MockBenchmarkRun imported from benchmark_run (unused-import)
W: 10, 0: Unused MockMachineManager imported from machine_manager (unused-import)
W: 11, 0: Unused MockResultsCache imported from results_cache (unused-import)
R:  1, 0: Missing "from __future__ import print_function" line (missing-print-function)