update marshmallow to >3.13.0

This is necessary so that `load_default` works. That's what triggered
that annoying deprecation warning about `metadata`.

See https://marshmallow.readthedocs.io/en/latest/changelog.html#id11

requirements.txt

@@ -33,7 +33,7 @@ uwsgi~=2.0.21
 uwsgitop~=0.11
 GitPython~=2.1.7
 mac-vendor-lookup~=0.1.11
-marshmallow~=3.11.1
+marshmallow~=3.20.1
 email-validator~=1.1.1
 sentry-sdk[Flask]~=1.29.2
 -e ./deps/wtforms-widgets/