style: code cleanups from Steven Burch (#29292)

* chore: update deprecated import from collections

* chore: remove outdated imports from markdown library

as it hasn't been supported since 2.0.3 and we're on 3.x.
This was deprecated at least as early as 2012!

* docs: add docstring and remove lint-amnesty to markdown plugin

* chore: remove deprecated etree import

* style: remove unnecessary-comprehension for sets

* style: resolve a number of amnestied pylint complaints

Co-authored-by: stvn <stvn@mit.edu>

common/test/acceptance/fixtures/base.py

@@ -56,7 +56,7 @@ class StudioApiFixture:
         Log in as a staff user, then return the cookies for the session (as a dict)
         Raises a `StudioApiLoginError` if the login fails.
         """
-        return {key: val for key, val in self.session.cookies.items()}  # lint-amnesty, pylint: disable=unnecessary-comprehension
+        return dict(self.session.cookies.items())
 
     @lazy
     def headers(self):

common/test/acceptance/fixtures/config.py

@@ -61,7 +61,7 @@ class ConfigModelFixture:
         Log in as a staff user, then return the cookies for the session (as a dict)
         Raises a `ConfigModelFixtureError` if the login fails.
         """
-        return {key: val for key, val in self.session.cookies.items()}  # lint-amnesty, pylint: disable=unnecessary-comprehension
+        return dict(self.session.cookies.items())
 
     @lazy
     def headers(self):