Make it possible to unvendor python-mimeparse

[vytas7]

As pointed out on #1420 (comment), some distributions and package managers may want to unvendor python-mimeparse, in order to reduce duplication, and because it carries a different licence, among other reasons.

Alternatively, we don't really make heavy use of python-mimeparse. Maybe we could instead reimplement & optimize the relevant function or two that we need? 🤔

[CaselIT]

since falconry is now maintaining the mimeparse project, an option may be to depend on that.