Many files lack copyright notices

[mhucka]

A number of files lack copyright statements at the top. Examples:

• _update_bazel_py_version.py
• src/py/BUILD
• src/py
• viz/to_json.py

We should add copyright statements where missing, and also implement an automated check (perhaps in the CI workflow) because it's easy for people (myself included) to forget.

[mhucka]

Resolved by #102