Use more appropriate install extras for CI:

- Use lighter-weight ``test`` extra for CI test runs.
  This should help us catch any issues with the test
  extra if any arise.

- Use both ``docs`` and ``test`` for docs test CI run.

tox.ini

@@ -33,7 +33,9 @@ commands=
     integration-ethtester: pytest {posargs:tests/integration/test_ethereum_tester.py}
     docs: make check-docs-ci
 deps =
-    .[dev]
+    .[test]
+    ; install both `docs` and `test` dependencies for the `docs` environment
+    docs: .[docs]
 passenv =
     GETH_BINARY
     GOROOT