Fix: add ledgereth to "testing" install target

Problem: `pip install .[testing]` is not enough to run all tests
successfully.

Solution: add missing packages to the "testing" install target.

setup.cfg

@@ -81,6 +81,7 @@ testing =
  flake8
  substrate-interface
  py-sr25519-bindings
+ ledgereth==0.9.0
 mqtt =
  aiomqtt<=0.1.3
  certifi