requirements: update elfi requirement from <0.8.1 to <0.8.3

Updates the requirements on [elfi](https://github.com/elfi-dev/elfi) to permit the latest version.
- [Release notes](https://github.com/elfi-dev/elfi/releases)
- [Commits](elfi-dev/elfi@0.2.2...v0.8.2)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 autograd<=1.3
 graph-scheduler>=0.2.0, <=1.0.0rc2
 dill<=0.32
-elfi<0.8.1
+elfi<0.8.3
 graphviz<0.18.0
 grpcio<1.42.0
 grpcio-tools<1.42.0