Add Darkgraylib @main to future test

The future compatibility test will now also update this package to the
tip of its `main` branch. This way we'll detect accidental
incompatibilities with our own library packages.

constraints-future.txt

@@ -4,5 +4,6 @@
 # up-to-date by adding entries for any packages we want to get early
 # incompatibility warnings for.
 black @ git+https://github.com/psf/black.git@main
+darkgraylib @ git+https://github.com/akaihola/darkgraylib.git@main
 flynt @ git+https://github.com/ikamensh/flynt.git@master
 isort @ git+https://github.com/PyCQA/isort.git@main