* fix: prefer PDF in materials_document(); refactor

* style: Black

* refactor: split urls entry into simpler pair

It seems Django cannot reverse a URL pattern that uses "|" to
combine options when one inclues a named variable and the other
does not.

* test: test extension choice

* fix: fix test failures

* refactor: get rid of io.open()

* refactor: reunite url patterns

Adds option for a trailing "/" when using an extension,
which was not previously supported but should be somewhere
between harmless and a feature.

ci: merge main to release (#8117)