requirements: update dill requirement from <0.3.8 to <0.3.9

Updates the requirements on [dill](https://github.com/uqfoundation/dill) to permit the latest version.
- [Release notes](https://github.com/uqfoundation/dill/releases)
- [Commits](uqfoundation/dill@dill-0.2.2...0.3.8)

---
updated-dependencies:
- dependency-name: dill
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -1,6 +1,6 @@
 autograd<1.7
 beartype<0.18.0
-dill<0.3.8
+dill<0.3.9
 fastkde>=1.0.24, <1.0.31
 graph-scheduler>=1.1.1, <1.3.0
 graphviz<0.21.0