Currently, copyright_headers.py uses the files' most recent git commit
year when updating or inserting a the copyright header. This is rather
silly as if you run the script again immediately after committing
copyright updates then then the script will update the copyright again
to the current year (where valid).

This change simply changes the script to update copyright to the current
year.

Note: Doesn't have to be src/seeder/test/, but makes testing easier as there is only one relevant file.

contrib/devtools/copyright_header.py src/seeder/test/

Should update from: // Copyright (c) 2019 The Bitcoin developers
To: // Copyright (c) 2019-2020 The Bitcoin developers
Repeat and it should make no change outputting Copyright up-to-date.

Repeat with directory with a python file.