Include platformdirs in dev requirements as a workaround

See 433074c681, this is the same cause. An older version of a
package being included in requirements files because setuptools injects
its vendored packages into sys.path and we use pip freeze to build lock
files. Then when you install two requirements files at the same time they
end up having conflicting versions. This at least means we include the
latest version, which will do until we move to a method of generating
lock files that just works off of the raw requirements file.

misc/requirements/requirements-dev.txt-raw

@@ -7,5 +7,9 @@ pyqt-builder
 build
 twine
 
+# Included to override setuptools' vendored version that is being included in
+# the lock file by pip freeze.
+platformdirs
+
 # Already included via test requirements
 #@ ignore: urllib3