Support Python 3.13's removal of mailcap

Python 3.13 removed the dead battery mailcap and it is now offered on
PyPi. This is temporarily workaround.

Related: #1632

pyproject.toml

@@ -33,6 +33,7 @@ dependencies = [
     "python-magic",
     "configobj>=4.7.0",
     "gpg>1.10.0",
+    "standard-mailcap; python_version>'3.12'"
 ]
 
 [project.optional-dependencies]