DOC: fix build failure with sphinx 7 and SOURCE_DATE_EPOCH
When SOURCE_DATE_EPOCH is set (which is the default when building Debian packages), Sphinx 7 [**requires** a space or comma after the year in 'copyright'](https://sources.debian.org/src/sphinx/7.1.2-2/sphinx/config.py/#L444).

[Debian bug report, with failure log](https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1042672).

- [not known to be reported here ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ n/a] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ CI says yes] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [n/a ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [no ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks @rebecca-palmer 