Building docs fails with Sphinx 7.2 and SOURCE_DATE_EPOCH environment variable

Nobody has claimed this yet.

Hi,
Currently your package fails to build from source in Debian with Sphinx 7.2:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1042586
The failure is caused by SOURCE_DATE_EPOCH environment variable added during build in Debian.
Please see [here](https://github.com/sphinx-doc/sphinx/pull/2503) and [here](https://github.com/sphinx-doc/sphinx/issues/11627) for reference.

Step to reproduce:
```
$ export SOURCE_DATE_EPOCH=1699611891; sphinx-build -b html python-flask-marshmallow-0.15.0/docs/ .

Running Sphinx v7.2.6
WARNING: html_static_path entry '_static' is placed inside outdir

Extension error (sphinx.config):
Handler for event 'config-inited' threw an exception (exception: string index out of range)
```
Offending lines:
https://github.com/marshmallow-code/flask-marshmallow/blob/c6392231851f624ca1e2ba2d864a67e4b7b106de/docs/conf.py#L37-L39

This change solves the issue:
```
copyright = "2014-{:%Y}, something".format(
dt.datetime.utcfromtimestamp(os.path.getmtime("../CHANGELOG.rst"))
)
```

Kind Regards

Start with docs/conf.py at the reported lines 37-39 and reproduce the failure using Sphinx 7.2 with SOURCE_DATE_EPOCH set. Verify the correction against the Debian-linked behavior, then run the documented sphinx-build command; done means the HTML documentation builds without the config-inited exception.

Written by the indexing model from the issue text.