Bump sphinx from 7.4.6 to 8.1.0 (#512)

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

aiosmtpd/docs/requirements.txt

@@ -1,6 +1,6 @@
 -r ../../requirements.txt
 
 ### Sphinx deps
-sphinx==7.4.6
+sphinx==8.1.0
 sphinx-autofixture==0.4.0
 sphinx_rtd_theme==3.0.1