Building docs fails with Sphinx 7.2 and SOURCE_DATE_EPOCH environment variable

[s3v-]

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 and here 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 <function correct_copyright_year at 0x7fd7fe000ae0> for event 'config-inited' threw an exception (exception: string index out of range)

Offending lines:

flask-marshmallow/docs/conf.py

Lines 37 to 39 in c639223

	copyright = "2014-{:%Y}".format(
	dt.datetime.utcfromtimestamp(os.path.getmtime("../CHANGELOG.rst"))
	)

This change solves the issue:

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

Kind Regards