TrigByteStreamTools: python fixes + unit test for updateMetadata script
- Move flake8 checks to scripts installation step
- Avoid using sublist operation on eformat.istream in dumpHLTContentInBS_run3 as it's very slow
- Add numEvents option to updateMetadata script
- Fix py2-py3 incompatibility in updateMetadata script (iteritems) discovered in ATR-21808
- Add unit test for updateMetadata script to avoid discovering such issues too late