Fix generation of notice file in CSV format

[jsoriano]

Fix generation of notice file in CSV format, and make ./dev-tools/generate_notice.py executable, so it can be directly executed and correct python interpreter will be taken from the shebang.

This script was failing with this error when used to generate the output in CSV format.

Get the licenses available from ['./vendor']
WARNING: No version information found for: github.com/prometheus/client_model
WARNING: No version information found for: github.com/Azure/azure-storage-blob-go
WARNING: No version information found for: github.com/Azure/azure-pipeline-go
WARNING: No version information found for: github.com/godror/godror/odpi
WARNING: No version information found for: github.com/go-ini/ini
Traceback (most recent call last):
  File "dev-tools/generate_notice.py", line 417, in <module>
    dependencies = create_notice(notice, args.beat, args.copyright, vendor_dirs, args.csvfile, overrides=overrides)
  File "dev-tools/generate_notice.py", line 227, in create_notice
    write_csv_file(csvwriter, dependencies)
  File "dev-tools/generate_notice.py", line 204, in write_csv_file
    csvwriter.writerow(["name", "url", "version", "revision", "license"])
TypeError: a bytes-like object is required, not 'str'

It could be reproduced with python3 ./dev-tools/generate_notice.py . --csv foo.csv.

[mikemadden42]

LGTM