doc/conf.py: if set, use SOURCE_DATE_EPOCH to set copyright year.

[vagrantc]

The build date of the software shouldn't really have any bearing on
the copyright dates, but by respecting SOURCE_DATE_EPOCH, it at least
limits this to the last time something in the source was changed.

https://reproducible-builds.org/specs/source-date-epoch/

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[vagrantc]

originally reported as https://bugs.debian.org/990339

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[jklymak]

Is this really an issue? Matplotlib source changes a lot more often than yearly.

[QuLogic]

SOURCE_DATE_EPOCH is set in package builds to make things more reproducible; we also obey it for timestamps in e.g., PDF files.

This needs to pass flake8 though.

[vagrantc]

Is this really an issue? Matplotlib source changes a lot more often than yearly.

One problem is if you build a version from 2019 in 2021, without changing anything in the source code, the documentation will incorrectly claim copyright for the year it was built(2021), not when the source was written(2019).

Another problem is if you build a version from 2019 in 2021, it will contain changes, even if you use the exact same toolchain for the two builds, so it is not bit-for-bit identical, requiring manual review of the differences rather than comparing for bit-for-bit identical files using a checksum.

Hope that helps explain it better!

[vagrantc]

flake8 issue should be fixed now.